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/api/collection.py

@@ -1,7 +1,6 @@
 """Collection API endpoints"""
 from fastapi import APIRouter, Query
 from typing import List, Optional
-from app.core.database import pool
 import json
 
 router = APIRouter()
@@ -15,17 +14,20 @@ async def get_collection_summary(
     Get collection summary by country and media type.
     Returns counts per country per media type.
     """
-    # Pool should be initialized on startup, but check just in case
-    if not pool:
-        from app.core.database import init_db
-        await init_db()
+    # Ensure pool is initialized
+    from app.core.database import init_db, pool as db_pool
+    await init_db()
+    
+    if db_pool is None:
+        from fastapi import HTTPException
+        raise HTTPException(status_code=503, detail="Database not available")
     
     # Parse types filter
     type_filter = []
     if types:
         type_filter = [t.strip() for t in types.split(",") if t.strip() in ["movie", "show", "music"]]
     
-    async with pool.connection() as conn:
+    async with db_pool.connection() as conn:
         async with conn.cursor() as cur:
             # Build query
             query = """