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.

cmd/migration/main.go

@@ -25,6 +25,11 @@ func MigrateDb() {
 		log.Fatal(err)
 		return
 	}
+	defer func() {
+		if sqlDB, e := db.DB(); e == nil {
+			_ = sqlDB.Close()
+		}
+	}()
 	tx := db.Begin()
 	defer func() {
 		if err == nil {