Refactor database connection handling in API endpoints

- Removed direct pool checks and replaced them with a centralized database initialization method in `init_db`.
- Updated API endpoints in `admin.py`, `collection.py`, `pins.py`, and `watched.py` to ensure the database connection pool is initialized before usage.
- Enhanced error handling to raise HTTP exceptions if the database is unavailable.
- Improved the `init_db` function in `database.py` to prevent multiple simultaneous initializations using an asyncio lock.

backend/app/services/sync.py

@@ -3,7 +3,6 @@ import httpx
 import logging
 from typing import Dict, List, Optional
 from app.core.config import settings
-from app.core.database import pool
 import json
 
 logger = logging.getLogger(__name__)
@@ -130,12 +129,12 @@ def extract_country_from_lidarr(artist: Dict) -> Optional[str]:
 async def upsert_media_item(source_kind: str, source_item_id: int, title: str, 
                            year: Optional[int], media_type: str, arr_raw: Dict):
     """Upsert a media item into the database"""
-    # Pool should be initialized on startup
-    if not pool:
-        from app.core.database import init_db
-        await init_db()
+    from app.core.database import init_db, pool as db_pool
+    await init_db()
+    if db_pool is None:
+        raise Exception("Database not available")
     
-    async with pool.connection() as conn:
+    async with db_pool.connection() as conn:
         async with conn.cursor() as cur:
             # Upsert media item
             query = """
@@ -273,12 +272,13 @@ async def sync_all_arrs() -> Dict:
         logger.error(f"Lidarr sync failed: {e}")
         results["lidarr"] = 0
     
-    # Update last sync time (pool should be initialized)
-    if not pool:
-        from app.core.database import init_db
-        await init_db()
+    # Update last sync time
+    from app.core.database import init_db, pool as db_pool
+    await init_db()
+    if db_pool is None:
+        raise Exception("Database not available")
     
-    async with pool.connection() as conn:
+    async with db_pool.connection() as conn:
         async with conn.cursor() as cur:
             for source_kind in ["radarr", "sonarr", "lidarr"]:
                 await cur.execute(