feat: implement caching for duty-related data and enhance performance

- Added a TTLCache class for in-memory caching of duty-related data, improving performance by reducing database queries.
- Integrated caching into the group duty pin functionality, allowing for efficient retrieval of message text and next shift end times.
- Introduced new methods to invalidate caches when relevant data changes, ensuring data consistency.
- Created a new Alembic migration to add indexes on the duties table for improved query performance.
- Updated tests to cover the new caching behavior and ensure proper functionality.

duty_teller/services/group_duty_pin_service.py

@@ -5,6 +5,7 @@ from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
 
 from sqlalchemy.orm import Session
 
+from duty_teller.cache import duty_pin_cache
 from duty_teller.db.repository import (
     get_current_duty,
     get_next_shift_end,
@@ -17,6 +18,31 @@ from duty_teller.i18n import t
 from duty_teller.utils.dates import parse_utc_iso
 
 
+def get_pin_refresh_data(
+    session: Session, chat_id: int, tz_name: str, lang: str = "en"
+) -> tuple[int | None, str, datetime | None]:
+    """Get all data needed for pin refresh in a single DB session.
+
+    Args:
+        session: DB session.
+        chat_id: Telegram chat id.
+        tz_name: Timezone name for display.
+        lang: Language code for i18n.
+
+    Returns:
+        (message_id, duty_message_text, next_shift_end_utc).
+        message_id is None if no pin record. next_shift_end_utc is naive UTC or None.
+    """
+    pin = get_group_duty_pin(session, chat_id)
+    message_id = pin.message_id if pin else None
+    if message_id is None:
+        return (None, t(lang, "duty.no_duty"), None)
+    now = datetime.now(timezone.utc)
+    text = get_duty_message_text(session, tz_name, lang)
+    next_end = get_next_shift_end(session, now)
+    return (message_id, text, next_end)
+
+
 def format_duty_message(duty, user, tz_name: str, lang: str = "en") -> str:
     """Build the text for the pinned duty message.
 
@@ -64,34 +90,31 @@ def format_duty_message(duty, user, tz_name: str, lang: str = "en") -> str:
 
 
 def get_duty_message_text(session: Session, tz_name: str, lang: str = "en") -> str:
-    """Get current duty from DB and return formatted message text.
-
-    Args:
-        session: DB session.
-        tz_name: Timezone name for display.
-        lang: Language code for i18n.
-
-    Returns:
-        Formatted duty message or "No duty" if none.
-    """
+    """Get current duty from DB and return formatted message text. Cached 90s."""
+    cache_key = ("duty_message_text", tz_name, lang)
+    text, found = duty_pin_cache.get(cache_key)
+    if found:
+        return text
     now = datetime.now(timezone.utc)
     result = get_current_duty(session, now)
     if result is None:
-        return t(lang, "duty.no_duty")
-    duty, user = result
-    return format_duty_message(duty, user, tz_name, lang)
+        text = t(lang, "duty.no_duty")
+    else:
+        duty, user = result
+        text = format_duty_message(duty, user, tz_name, lang)
+    duty_pin_cache.set(cache_key, text)
+    return text
 
 
 def get_next_shift_end_utc(session: Session) -> datetime | None:
-    """Return next shift end as naive UTC datetime for job scheduling.
-
-    Args:
-        session: DB session.
-
-    Returns:
-        Next shift end (naive UTC) or None.
-    """
-    return get_next_shift_end(session, datetime.now(timezone.utc))
+    """Return next shift end as naive UTC datetime for job scheduling. Cached 90s."""
+    cache_key = ("next_shift_end",)
+    value, found = duty_pin_cache.get(cache_key)
+    if found:
+        return value
+    result = get_next_shift_end(session, datetime.now(timezone.utc))
+    duty_pin_cache.set(cache_key, result)
+    return result
 
 
 def save_pin(session: Session, chat_id: int, message_id: int) -> None: