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/core/database.py

@@ -4,28 +4,52 @@ from psycopg_pool import AsyncConnectionPool
 from app.core.config import settings
 from typing import Optional
 import logging
+import asyncio
 
 logger = logging.getLogger(__name__)
 
 # Connection pool
 pool: Optional[AsyncConnectionPool] = None
+_init_lock = asyncio.Lock()
+_initializing = False
 
 
 async def init_db():
     """Initialize database connection pool"""
-    global pool
-    try:
-        pool = AsyncConnectionPool(
-            conninfo=settings.database_url,
-            min_size=1,
-            max_size=10,
-            open=False,
-        )
-        await pool.open()
-        logger.info("Database connection pool initialized")
-    except Exception as e:
-        logger.error(f"Failed to initialize database pool: {e}")
-        raise
+    global pool, _initializing
+    
+    # If already initialized, return
+    if pool is not None:
+        return
+    
+    # Use lock to prevent multiple simultaneous initializations
+    async with _init_lock:
+        # Double-check after acquiring lock
+        if pool is not None:
+            return
+        
+        if _initializing:
+            # Wait for other initialization to complete
+            while _initializing:
+                await asyncio.sleep(0.1)
+            return
+        
+        _initializing = True
+        try:
+            pool = AsyncConnectionPool(
+                conninfo=settings.database_url,
+                min_size=1,
+                max_size=10,
+                open=False,
+            )
+            await pool.open()
+            logger.info("Database connection pool initialized")
+        except Exception as e:
+            logger.error(f"Failed to initialize database pool: {e}")
+            pool = None
+            raise
+        finally:
+            _initializing = False
 
 
 async def close_db():