fix(db): stabilize connection settings and explicitly define AppConfig table name

- Set SQL connection idle limit to 1 to reduce resource usage
- Explicitly define AppConfig table name to prevent GORM pluralization issues

internal/db/DB.go

@@ -80,6 +80,7 @@ func Connect(dbpath string) (*gorm.DB, error) {
 		return nil, fmt.Errorf("could not get underlying sql.DB: %w", err)
 	}
 	sqlDB.SetMaxOpenConns(1)
+	sqlDB.SetMaxIdleConns(1)
 
 	// ── Pragmas ───────────────────────────────────────────────────────────
 	pragmas := []string{