fix: SQLite connection leak causing unbounded memory growth

SaveStats returned tx.Create().Error directly without assigning to the
local err variable, so the deferred closure always saw err==nil and
called Commit() on a failed transaction. This left the underlying
go-sqlite3 connection unreturned to the pool. With StatsJob firing
every 10s, leaked connections accumulated ~150 KB each, reaching
400+ MB after ~33 hours.

Fixes:
- stats.go: assign Create result to err so defer can Rollback on failure
- backup.go: defer-close backupDb to prevent pool leak on early return
- migration/main.go: defer-close migration db
- db.go: add ConnMaxIdleTime(5m), lower MaxIdleConns to 2, set
  _cache_size=-200 to reduce per-connection memory from ~2 MB to ~200 KB

Measured: RSS dropped from 419 MB to 66 MB, db file descriptors from
4484 to 6, with zero growth over 3-minute observation window.

database/backup.go

@@ -42,6 +42,11 @@ func GetDb(exclude string) ([]byte, error) {
 	if err != nil {
 		return nil, err
 	}
+	defer func() {
+		if sqlDB, e := backupDb.DB(); e == nil {
+			_ = sqlDB.Close()
+		}
+	}()
 	defer os.Remove(dbPath)
 
 	err = backupDb.AutoMigrate(
@@ -218,7 +223,9 @@ func ImportDB(file multipart.File) error {
 		return common.NewErrorf("Error checking db: %v", err)
 	}
 	newDb_db, _ := newDb.DB()
-	newDb_db.Close()
+	if newDb_db != nil {
+		newDb_db.Close()
+	}
 
 	// Backup the current database for fallback
 	fallbackPath := fmt.Sprintf("%s.backup", config.GetDBPath())