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/handlers/common.py

@@ -3,22 +3,35 @@
 import asyncio
 
 import duty_teller.config as config
+from duty_teller.cache import is_admin_cache
 from duty_teller.db.repository import is_admin_for_telegram_user
 from duty_teller.db.session import session_scope
 
 
 async def is_admin_async(telegram_user_id: int) -> bool:
-    """Check if Telegram user is admin (username or phone). Runs DB check in executor.
+    """Check if Telegram user is admin. Cached 60s. Invalidated on set_user_role.
 
     Args:
         telegram_user_id: Telegram user id.
 
     Returns:
-        True if user is in ADMIN_USERNAMES or their stored phone is in ADMIN_PHONES.
+        True if user is admin (DB role or env fallback).
     """
+    cache_key = ("is_admin", telegram_user_id)
+    value, found = is_admin_cache.get(cache_key)
+    if found:
+        return value
 
     def _check() -> bool:
         with session_scope(config.DATABASE_URL) as session:
             return is_admin_for_telegram_user(session, telegram_user_id)
 
-    return await asyncio.get_running_loop().run_in_executor(None, _check)
+    result = await asyncio.get_running_loop().run_in_executor(None, _check)
+    is_admin_cache.set(cache_key, result)
+    return result
+
+
+def invalidate_is_admin_cache(telegram_user_id: int | None) -> None:
+    """Invalidate is_admin cache for user. Call after set_user_role."""
+    if telegram_user_id is not None:
+        is_admin_cache.invalidate(("is_admin", telegram_user_id))