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>

conf/configuration.go

@@ -87,6 +87,7 @@ type configOptions struct {
 	Prometheus                      prometheusOptions
 	Scanner                         scannerOptions
 	Jukebox                         jukeboxOptions
+	Backup                          backupOptions
 
 	Agents       string
 	LastFM       lastfmOptions
@@ -153,6 +154,12 @@ type jukeboxOptions struct {
 	AdminOnly bool
 }
 
+type backupOptions struct {
+	Count    int
+	Path     string
+	Schedule string
+}
+
 var (
 	Server = &configOptions{}
 	hooks  []func()
@@ -194,6 +201,14 @@ func Load() {
 		Server.DbPath = filepath.Join(Server.DataFolder, consts.DefaultDbPath)
 	}
 
+	if Server.Backup.Path != "" {
+		err = os.MkdirAll(Server.Backup.Path, os.ModePerm)
+		if err != nil {
+			_, _ = fmt.Fprintln(os.Stderr, "FATAL: Error creating backup path:", "path", Server.Backup.Path, err)
+			os.Exit(1)
+		}
+	}
+
 	log.SetLevelString(Server.LogLevel)
 	log.SetLogLevels(Server.DevLogLevels)
 	log.SetLogSourceLine(Server.DevLogSourceLine)
@@ -203,6 +218,10 @@ func Load() {
 		os.Exit(1)
 	}
 
+	if err := validateBackupSchedule(); err != nil {
+		os.Exit(1)
+	}
+
 	if Server.BaseURL != "" {
 		u, err := url.Parse(Server.BaseURL)
 		if err != nil {
@@ -264,15 +283,35 @@ func validateScanSchedule() error {
 		Server.ScanSchedule = ""
 		return nil
 	}
-	if _, err := time.ParseDuration(Server.ScanSchedule); err == nil {
-		Server.ScanSchedule = "@every " + Server.ScanSchedule
+	var err error
+	Server.ScanSchedule, err = validateSchedule(Server.ScanSchedule, "ScanSchedule")
+	return err
+}
+
+func validateBackupSchedule() error {
+	if Server.Backup.Path == "" || Server.Backup.Schedule == "" || Server.Backup.Count == 0 {
+		Server.Backup.Schedule = ""
+		return nil
+	}
+
+	var err error
+	Server.Backup.Schedule, err = validateSchedule(Server.Backup.Schedule, "BackupSchedule")
+
+	return err
+}
+
+func validateSchedule(schedule, field string) (string, error) {
+	if _, err := time.ParseDuration(schedule); err == nil {
+		schedule = "@every " + schedule
 	}
 	c := cron.New()
-	_, err := c.AddFunc(Server.ScanSchedule, func() {})
+	id, err := c.AddFunc(schedule, func() {})
 	if err != nil {
-		log.Error("Invalid ScanSchedule. Please read format spec at https://pkg.go.dev/github.com/robfig/cron#hdr-CRON_Expression_Format", "schedule", Server.ScanSchedule, err)
+		log.Error(fmt.Sprintf("Invalid %s. Please read format spec at https://pkg.go.dev/github.com/robfig/cron#hdr-CRON_Expression_Format", field), "schedule", field, err)
+	} else {
+		c.Remove(id)
 	}
-	return err
+	return schedule, err
 }
 
 // AddHook is used to register initialization code that should run as soon as the config is loaded
@@ -365,6 +404,10 @@ func init() {
 
 	viper.SetDefault("httpsecurityheaders.customframeoptionsvalue", "DENY")
 
+	viper.SetDefault("backup.path", "")
+	viper.SetDefault("backup.schedule", "")
+	viper.SetDefault("backup.count", 0)
+
 	// DevFlags. These are used to enable/disable debugging and incomplete features
 	viper.SetDefault("devlogsourceline", false)
 	viper.SetDefault("devenableprofiler", false)