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/config.py

@@ -26,7 +26,8 @@ class PerformanceSettings(BaseModel):
     """Performance tuning settings"""
     max_concurrent_thumbnails: int = 10
     cache_ttl: int = 3600
-    db_pool_size: int = 20
+    db_pool_size: int = 5
+    db_pool_max_overflow: int = 5
     db_pool_recycle: int = 3600
 
 class EmbedderSettings(BaseModel):