Bound connection pool to prevent max_connections exhaustion

pool_size=5, max_overflow=10 per app (6 apps = 90 max connections).
Previously pool_size=0 meant unlimited connections, causing
asyncpg.TooManyConnectionsError under concurrent load.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db/session.py

@@ -15,7 +15,8 @@ _engine = create_async_engine(
     future=True,
     echo=False,
     pool_pre_ping=True,
-    pool_size=0,  # 0 = unlimited (NullPool equivalent for asyncpg)
+    pool_size=5,
+    max_overflow=10,
 )
 
 _Session = async_sessionmaker(