chore: add changelog and documentation updates

- Created a new `CHANGELOG.md` file to document all notable changes to the project, adhering to the Keep a Changelog format.
- Updated `CONTRIBUTING.md` to include instructions for building and previewing documentation using MkDocs.
- Added `mkdocs.yml` configuration for documentation generation, including navigation structure and theme settings.
- Enhanced various documentation files, including API reference, architecture overview, configuration reference, and runbook, to provide comprehensive guidance for users and developers.
- Included new sections in the README for changelog and documentation links, improving accessibility to project information.

duty_teller/db/repository.py

@@ -11,7 +11,15 @@ from duty_teller.db.models import User, Duty, GroupDutyPin, CalendarSubscription
 
 
 def get_user_by_telegram_id(session: Session, telegram_user_id: int) -> User | None:
-    """Find user by telegram_user_id. Returns None if not found (no creation)."""
+    """Find user by Telegram user ID.
+
+    Args:
+        session: DB session.
+        telegram_user_id: Telegram user id.
+
+    Returns:
+        User or None if not found. Does not create a user.
+    """
     return session.query(User).filter(User.telegram_user_id == telegram_user_id).first()
 
 
@@ -23,10 +31,22 @@ def get_or_create_user(
     first_name: str | None = None,
     last_name: str | None = None,
 ) -> User:
-    """
-    Get or create user by telegram_user_id. On create, name comes from Telegram.
-    On update: username is always synced; full_name/first_name/last_name are only
-    updated if name_manually_edited is False (otherwise keep existing display name).
+    """Get or create user by Telegram user ID.
+
+    On create, name fields come from Telegram. On update: username is always
+    synced; full_name, first_name, last_name are updated only if
+    name_manually_edited is False (otherwise existing display name is kept).
+
+    Args:
+        session: DB session.
+        telegram_user_id: Telegram user id.
+        full_name: Display full name.
+        username: Telegram username (optional).
+        first_name: Telegram first name (optional).
+        last_name: Telegram last name (optional).
+
+    Returns:
+        User instance (created or updated).
     """
     user = get_user_by_telegram_id(session, telegram_user_id)
     if user:
@@ -53,9 +73,16 @@ def get_or_create_user(
 
 
 def get_or_create_user_by_full_name(session: Session, full_name: str) -> User:
-    """
-    Find user by exact full_name or create one with telegram_user_id=None (for duty-schedule import).
-    New users get name_manually_edited=True since the name comes from import, not Telegram.
+    """Find user by exact full_name or create one (for duty-schedule import).
+
+    New users have telegram_user_id=None and name_manually_edited=True.
+
+    Args:
+        session: DB session.
+        full_name: Exact full name to match or set.
+
+    Returns:
+        User instance (existing or newly created).
     """
     user = session.query(User).filter(User.full_name == full_name).first()
     if user:
@@ -81,10 +108,20 @@ def update_user_display_name(
     first_name: str | None = None,
     last_name: str | None = None,
 ) -> User | None:
-    """
-    Update display name for user by telegram_user_id and set name_manually_edited=True.
-    Use from API or admin when name is changed manually; subsequent get_or_create_user
-    will not overwrite these fields. Returns User or None if not found.
+    """Update display name and set name_manually_edited=True.
+
+    Use from API or admin when name is changed manually; subsequent
+    get_or_create_user will not overwrite these fields.
+
+    Args:
+        session: DB session.
+        telegram_user_id: Telegram user id.
+        full_name: New full name.
+        first_name: New first name (optional).
+        last_name: New last name (optional).
+
+    Returns:
+        Updated User or None if not found.
     """
     user = session.query(User).filter(User.telegram_user_id == telegram_user_id).first()
     if not user:
@@ -104,7 +141,17 @@ def delete_duties_in_range(
     from_date: str,
     to_date: str,
 ) -> int:
-    """Delete all duties of the user that overlap [from_date, to_date] (YYYY-MM-DD). Returns count deleted."""
+    """Delete all duties of the user that overlap the given date range.
+
+    Args:
+        session: DB session.
+        user_id: User id.
+        from_date: Start date YYYY-MM-DD.
+        to_date: End date YYYY-MM-DD.
+
+    Returns:
+        Number of duties deleted.
+    """
     to_next = (
         datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
     ).strftime("%Y-%m-%d")
@@ -124,7 +171,16 @@ def get_duties(
     from_date: str,
     to_date: str,
 ) -> list[tuple[Duty, str]]:
-    """Return list of (Duty, full_name) overlapping the given date range."""
+    """Return duties overlapping the given date range with user full_name.
+
+    Args:
+        session: DB session.
+        from_date: Start date YYYY-MM-DD.
+        to_date: End date YYYY-MM-DD.
+
+    Returns:
+        List of (Duty, full_name) tuples.
+    """
     to_date_next = (
         datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
     ).strftime("%Y-%m-%d")
@@ -142,7 +198,17 @@ def get_duties_for_user(
     from_date: str,
     to_date: str,
 ) -> list[tuple[Duty, str]]:
-    """Return list of (Duty, full_name) for the given user overlapping the date range."""
+    """Return duties for one user overlapping the date range.
+
+    Args:
+        session: DB session.
+        user_id: User id.
+        from_date: Start date YYYY-MM-DD.
+        to_date: End date YYYY-MM-DD.
+
+    Returns:
+        List of (Duty, full_name) tuples.
+    """
     to_date_next = (
         datetime.fromisoformat(to_date + "T00:00:00") + timedelta(days=1)
     ).strftime("%Y-%m-%d")
@@ -164,9 +230,17 @@ def _token_hash(token: str) -> str:
 
 
 def create_calendar_token(session: Session, user_id: int) -> str:
-    """
-    Create a new calendar subscription token for the user.
-    Removes any existing tokens for this user. Returns the raw token string.
+    """Create a new calendar subscription token for the user.
+
+    Any existing tokens for this user are removed. The raw token is returned
+    only once (not stored in plain text).
+
+    Args:
+        session: DB session.
+        user_id: User id.
+
+    Returns:
+        Raw token string (e.g. for URL /api/calendar/ical/{token}.ics).
     """
     session.query(CalendarSubscriptionToken).filter(
         CalendarSubscriptionToken.user_id == user_id
@@ -185,9 +259,16 @@ def create_calendar_token(session: Session, user_id: int) -> str:
 
 
 def get_user_by_calendar_token(session: Session, token: str) -> User | None:
-    """
-    Find user by calendar subscription token. Uses constant-time comparison.
-    Returns None if token is invalid or not found.
+    """Find user by calendar subscription token.
+
+    Uses constant-time comparison to avoid timing leaks.
+
+    Args:
+        session: DB session.
+        token: Raw token from URL.
+
+    Returns:
+        User or None if token is invalid or not found.
     """
     token_hash_val = _token_hash(token)
     row = (
@@ -211,7 +292,18 @@ def insert_duty(
     end_at: str,
     event_type: str = "duty",
 ) -> Duty:
-    """Create a duty. start_at and end_at must be UTC, ISO 8601 with Z."""
+    """Create a duty record.
+
+    Args:
+        session: DB session.
+        user_id: User id.
+        start_at: Start time UTC, ISO 8601 with Z (e.g. 2025-01-15T09:00:00Z).
+        end_at: End time UTC, ISO 8601 with Z.
+        event_type: One of "duty", "unavailable", "vacation". Default "duty".
+
+    Returns:
+        Created Duty instance.
+    """
     duty = Duty(
         user_id=user_id,
         start_at=start_at,
@@ -225,7 +317,15 @@ def insert_duty(
 
 
 def get_current_duty(session: Session, at_utc: datetime) -> tuple[Duty, User] | None:
-    """Return the duty (and user) for which start_at <= at_utc < end_at, event_type='duty'."""
+    """Return the duty and user active at the given UTC time (event_type='duty').
+
+    Args:
+        session: DB session.
+        at_utc: Point in time (timezone-aware or naive UTC).
+
+    Returns:
+        (Duty, User) or None if no duty at that time.
+    """
     from datetime import timezone
 
     if at_utc.tzinfo is not None:
@@ -247,7 +347,15 @@ def get_current_duty(session: Session, at_utc: datetime) -> tuple[Duty, User] |
 
 
 def get_next_shift_end(session: Session, after_utc: datetime) -> datetime | None:
-    """Return the end_at of the current duty or of the next duty. Naive UTC."""
+    """Return the end_at of the current or next duty (event_type='duty').
+
+    Args:
+        session: DB session.
+        after_utc: Point in time (timezone-aware or naive UTC).
+
+    Returns:
+        End datetime (naive UTC) or None if no current or future duty.
+    """
     from datetime import timezone
 
     if after_utc.tzinfo is not None:
@@ -280,14 +388,31 @@ def get_next_shift_end(session: Session, after_utc: datetime) -> datetime | None
 
 
 def get_group_duty_pin(session: Session, chat_id: int) -> GroupDutyPin | None:
-    """Get the pinned message record for a chat, if any."""
+    """Get the pinned duty message record for a chat.
+
+    Args:
+        session: DB session.
+        chat_id: Telegram chat id.
+
+    Returns:
+        GroupDutyPin or None.
+    """
     return session.query(GroupDutyPin).filter(GroupDutyPin.chat_id == chat_id).first()
 
 
 def save_group_duty_pin(
     session: Session, chat_id: int, message_id: int
 ) -> GroupDutyPin:
-    """Save or update the pinned message for a chat."""
+    """Save or update the pinned duty message for a chat.
+
+    Args:
+        session: DB session.
+        chat_id: Telegram chat id.
+        message_id: Message id to pin/update.
+
+    Returns:
+        GroupDutyPin instance (created or updated).
+    """
     pin = session.query(GroupDutyPin).filter(GroupDutyPin.chat_id == chat_id).first()
     if pin:
         pin.message_id = message_id
@@ -300,13 +425,27 @@ def save_group_duty_pin(
 
 
 def delete_group_duty_pin(session: Session, chat_id: int) -> None:
-    """Remove the pinned message record when the bot leaves the group."""
+    """Remove the pinned duty message record for the chat (e.g. when bot leaves group).
+
+    Args:
+        session: DB session.
+        chat_id: Telegram chat id.
+    """
     session.query(GroupDutyPin).filter(GroupDutyPin.chat_id == chat_id).delete()
     session.commit()
 
 
 def get_all_group_duty_pin_chat_ids(session: Session) -> list[int]:
-    """Return all chat_ids that have a pinned duty message (for restoring jobs on startup)."""
+    """Return all chat_ids that have a pinned duty message.
+
+    Used to restore update jobs on bot startup.
+
+    Args:
+        session: DB session.
+
+    Returns:
+        List of chat ids.
+    """
     rows = session.query(GroupDutyPin.chat_id).all()
     return [r[0] for r in rows]
 
@@ -314,7 +453,16 @@ def get_all_group_duty_pin_chat_ids(session: Session) -> list[int]:
 def set_user_phone(
     session: Session, telegram_user_id: int, phone: str | None
 ) -> User | None:
-    """Set phone for user by telegram_user_id. Returns User or None if not found."""
+    """Set or clear phone for user by Telegram user id.
+
+    Args:
+        session: DB session.
+        telegram_user_id: Telegram user id.
+        phone: Phone string or None to clear.
+
+    Returns:
+        Updated User or None if not found.
+    """
     user = session.query(User).filter(User.telegram_user_id == telegram_user_id).first()
     if not user:
         return None