fix: reduce db pool size to prevent connection exhaustion, sort grouped views by count

Pool was 20+10 overflow per engine; uvicorn --reload leaked pools until
Postgres hit max_connections=100. Reduced to 5+5 with pool_pre_ping.

Grouped views (tags, people, colors, rated) now sort cards largest-first.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/database.py

@@ -73,7 +73,9 @@ else:
         settings.database_url,
         echo=False,
         pool_size=settings.performance.db_pool_size,
+        max_overflow=settings.performance.db_pool_max_overflow,
         pool_recycle=settings.performance.db_pool_recycle,
+        pool_pre_ping=True,
     )
 
 # Create async session factory