Improve request logging and make health check lightweight

- Log slow requests (>1s) at WARNING level with slow=true flag
- Log request timeouts at WARNING level in TimeoutMiddleware
- Replace heavy GetStatsContext with lightweight Ping in health check
- Add Ping method to database interface (SELECT 1)

internal/database/interface.go

@@ -88,6 +88,7 @@ type Store interface {
 	Vacuum(ctx context.Context) error
 	Analyze(ctx context.Context) error
 	Checkpoint(ctx context.Context) error
+	Ping(ctx context.Context) error
 }
 
 // Ensure Database implements Store