feat: add calendar subscription token functionality and ICS generation

- Introduced a new database model for calendar subscription tokens, allowing users to generate unique tokens for accessing their personal calendar.
- Implemented API endpoint to return ICS files containing only the subscribing user's duties, enhancing user experience with personalized calendar access.
- Added utility functions for generating ICS files from user duties, ensuring proper formatting and timezone handling.
- Updated command handlers to support the new calendar link feature, providing users with easy access to their personal calendar subscriptions.
- Included unit tests for the new functionality, ensuring reliability and correctness of token generation and ICS file creation.

duty_teller/api/app.py

@@ -1,10 +1,12 @@
 """FastAPI app: /api/duties and static webapp."""
 
 import logging
+from datetime import date, timedelta
 
 import duty_teller.config as config
 from fastapi import Depends, FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import Response
 from fastapi.staticfiles import StaticFiles
 from sqlalchemy.orm import Session
 
@@ -15,6 +17,8 @@ from duty_teller.api.dependencies import (
     get_validated_dates,
     require_miniapp_username,
 )
+from duty_teller.api.personal_calendar_ics import build_personal_ics
+from duty_teller.db.repository import get_duties_for_user, get_user_by_calendar_token
 from duty_teller.db.schemas import CalendarEvent, DutyWithUser
 
 log = logging.getLogger(__name__)
@@ -57,6 +61,31 @@ def list_calendar_events(
     return [CalendarEvent(date=e["date"], summary=e["summary"]) for e in events]
 
 
+@app.get("/api/calendar/ical/{token}.ics")
+def get_personal_calendar_ical(
+    token: str,
+    session: Session = Depends(get_db_session),
+) -> Response:
+    """
+    Return ICS calendar with only the subscribing user's duties.
+    No Telegram auth; access is by secret token in the URL.
+    """
+    user = get_user_by_calendar_token(session, token)
+    if user is None:
+        return Response(status_code=404, content="Not found")
+    today = date.today()
+    from_date = (today - timedelta(days=365)).strftime("%Y-%m-%d")
+    to_date = (today + timedelta(days=365 * 2)).strftime("%Y-%m-%d")
+    duties_with_name = get_duties_for_user(
+        session, user.id, from_date=from_date, to_date=to_date
+    )
+    ics_bytes = build_personal_ics(duties_with_name)
+    return Response(
+        content=ics_bytes,
+        media_type="text/calendar; charset=utf-8",
+    )
+
+
 webapp_path = config.PROJECT_ROOT / "webapp"
 if webapp_path.is_dir():
     app.mount("/app", StaticFiles(directory=str(webapp_path), html=True), name="webapp")