feat: enhance error handling and configuration validation

- Added a global exception handler to log unhandled exceptions and return a generic 500 JSON response without exposing details to the client.
- Updated the configuration to validate the `DATABASE_URL` format, ensuring it starts with `sqlite://` or `postgresql://`, and log warnings for invalid formats.
- Introduced safe parsing for numeric environment variables (`HTTP_PORT`, `INIT_DATA_MAX_AGE_SECONDS`) with defaults on invalid values, including logging warnings for out-of-range values.
- Enhanced the duty schedule parser to enforce limits on the number of schedule rows and the length of full names and duty strings, raising appropriate errors when exceeded.
- Updated internationalization messages to include generic error responses for import failures and parsing issues, improving user experience.
- Added unit tests to verify the new error handling and configuration validation behaviors.

duty_teller/db/repository.py

@@ -7,6 +7,7 @@ from datetime import datetime, timezone
 from sqlalchemy.orm import Session
 
 import duty_teller.config as config
+from duty_teller.db.schemas import DUTY_EVENT_TYPES
 from duty_teller.db.models import (
     User,
     Duty,
@@ -201,14 +202,19 @@ def get_or_create_user(
     return user
 
 
-def get_or_create_user_by_full_name(session: Session, full_name: str) -> User:
+def get_or_create_user_by_full_name(
+    session: Session, full_name: str, *, commit: bool = True
+) -> User:
     """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.
+    When commit=False, caller is responsible for committing (e.g. single commit
+    per import in run_import).
 
     Args:
         session: DB session.
         full_name: Exact full name to match or set.
+        commit: If True, commit immediately. If False, caller commits.
 
     Returns:
         User instance (existing or newly created).
@@ -225,8 +231,11 @@ def get_or_create_user_by_full_name(session: Session, full_name: str) -> User:
         name_manually_edited=True,
     )
     session.add(user)
-    session.commit()
-    session.refresh(user)
+    if commit:
+        session.commit()
+        session.refresh(user)
+    else:
+        session.flush()  # Assign id so caller can use user.id before commit
     return user
 
 
@@ -447,11 +456,13 @@ def insert_duty(
         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".
+        event_type: One of "duty", "unavailable", "vacation". Invalid values are stored as "duty".
 
     Returns:
         Created Duty instance.
     """
+    if event_type not in DUTY_EVENT_TYPES:
+        event_type = "duty"
     duty = Duty(
         user_id=user_id,
         start_at=start_at,