database: create shared database package

Extract common database functionality from api/ntpdb and monitor/ntpdb
into shared common/database package:

- Dynamic connector pattern with configuration loading
- Configurable connection pool management (API: 25/10, Monitor: 10/5)
- Optional Prometheus metrics integration
- Generic transaction helpers with proper error handling
- Unified interfaces compatible with SQLC-generated code

Foundation for migration to eliminate ~200 lines of duplicate code.

database/transaction.go

@@ -41,11 +41,12 @@ func WithTransaction[Q TX](ctx context.Context, db DB[Q], fn func(ctx context.Co
 		return err
 	}
 
-	if err := tx.Commit(ctx); err != nil {
+	err = tx.Commit(ctx)
+	committed = true // Mark as committed regardless of commit success/failure
+	if err != nil {
 		return fmt.Errorf("failed to commit transaction: %w", err)
 	}
 
-	committed = true
 	return nil
 }