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/api/app.py

@@ -3,6 +3,7 @@
 import logging
 import re
 from datetime import date, timedelta
+
 import duty_teller.config as config
 from fastapi import Depends, FastAPI, Request
 from fastapi.middleware.cors import CORSMiddleware
@@ -18,6 +19,7 @@ from duty_teller.api.dependencies import (
     require_miniapp_username,
 )
 from duty_teller.api.personal_calendar_ics import build_personal_ics, build_team_ics
+from duty_teller.cache import ics_calendar_cache
 from duty_teller.db.repository import (
     get_duties,
     get_duties_for_user,
@@ -116,14 +118,18 @@ def get_team_calendar_ical(
     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")
-    all_duties = get_duties(session, from_date=from_date, to_date=to_date)
-    duties_duty_only = [
-        (d, name) for d, name in all_duties if (d.event_type or "duty") == "duty"
-    ]
-    ics_bytes = build_team_ics(duties_duty_only)
+    cache_key = ("team_ics",)
+    ics_bytes, found = ics_calendar_cache.get(cache_key)
+    if 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")
+        all_duties = get_duties(session, from_date=from_date, to_date=to_date)
+        duties_duty_only = [
+            (d, name) for d, name in all_duties if (d.event_type or "duty") == "duty"
+        ]
+        ics_bytes = build_team_ics(duties_duty_only)
+        ics_calendar_cache.set(cache_key, ics_bytes)
     return Response(
         content=ics_bytes,
         media_type="text/calendar; charset=utf-8",
@@ -151,13 +157,17 @@ def get_personal_calendar_ical(
     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, event_types=["duty"]
-    )
-    ics_bytes = build_personal_ics(duties_with_name)
+    cache_key = ("personal_ics", user.id)
+    ics_bytes, found = ics_calendar_cache.get(cache_key)
+    if 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, event_types=["duty"]
+        )
+        ics_bytes = build_personal_ics(duties_with_name)
+        ics_calendar_cache.set(cache_key, ics_bytes)
     return Response(
         content=ics_bytes,
         media_type="text/calendar; charset=utf-8",