feat(server): provide native backup/restore mechanism (#3194)

* [enhancement]: Provide native backup/restore mechanism

- db.go: add Backup/Restore functions that utilize Sqlite's built-in online backup mechanism
- support automatic backup with schedule, limit number of files
- provide commands to manually backup/restore Navidrome

Notes:
`Step(-1)` results in a read-only lock being held for the entire duration of the backup.
This will block out any other write operation (and may hold additional locks.
An alternate implementation that doesn't block but instead retries is available at https://www.sqlite.org/backup.html#:~:text=of%20a%20Running-,database,-%2F*%0A**%20Perform%20an%20online (easily adaptable to go), but has the potential problem of continually getting restarted by background writes.

Additionally, the restore should still only be called when Navidrome is offline, as the restore process does not run migrations that are missing.

* remove empty line

* add more granular backup schedule

* do not remove files when bypass is set

* move prune

* refactor: small nitpicks

* change commands and flags

* tests, return path from backup

* refactor: small nitpicks

---------

Co-authored-by: Deluan <deluan@navidrome.org>

cmd/root.go

@@ -79,6 +79,7 @@ func runNavidrome(ctx context.Context) {
 	g.Go(startScheduler(ctx))
 	g.Go(startPlaybackServer(ctx))
 	g.Go(schedulePeriodicScan(ctx))
+	g.Go(schedulePeriodicBackup(ctx))
 
 	if err := g.Wait(); err != nil {
 		log.Error("Fatal error in Navidrome. Aborting", err)
@@ -153,6 +154,42 @@ func schedulePeriodicScan(ctx context.Context) func() error {
 	}
 }
 
+func schedulePeriodicBackup(ctx context.Context) func() error {
+	return func() error {
+		schedule := conf.Server.Backup.Schedule
+		if schedule == "" {
+			log.Warn("Periodic backup is DISABLED")
+			return nil
+		}
+
+		database := db.Db()
+		schedulerInstance := scheduler.GetInstance()
+
+		log.Info("Scheduling periodic backup", "schedule", schedule)
+		err := schedulerInstance.Add(schedule, func() {
+			start := time.Now()
+			path, err := database.Backup(ctx)
+			elapsed := time.Since(start)
+			if err != nil {
+				log.Error(ctx, "Error backing up database", "elapsed", elapsed, err)
+				return
+			}
+			log.Info(ctx, "Backup complete", "elapsed", elapsed, "path", path)
+
+			count, err := database.Prune(ctx)
+			if err != nil {
+				log.Error(ctx, "Error pruning database", "error", err)
+			} else if count > 0 {
+				log.Info(ctx, "Successfully pruned old files", "count", count)
+			} else {
+				log.Info(ctx, "No backups pruned")
+			}
+		})
+
+		return err
+	}
+}
+
 // startScheduler starts the Navidrome scheduler, which is used to run periodic tasks.
 func startScheduler(ctx context.Context) func() error {
 	return func() error {