feat: configuration backup management with manual and auto backup

Add backup/restore functionality for the SQLite database. Users can
trigger manual backups, configure automatic backups on an interval
with retention policies, list/download/delete backups, and restore
from any backup.

- Backup engine using VACUUM INTO (safe with WAL mode)
- Backup metadata tracked in DB, files stored in DATA_DIR/backups/
- Settings: backup_enabled, backup_interval_hours, backup_retention_count
- API: POST/GET/DELETE /api/backups, download, restore endpoints
- Autobackup via cron scheduler with configurable interval
- Retention: prune on startup, after each backup (manual and auto)
- Orphan cleanup: removes backup files without metadata on startup
- Restore: replaces DB and triggers graceful server shutdown
- Settings UI: /settings/backup with toggle, interval, retention config
- Backup list with download, delete, restore actions
- i18n: English and Russian translations

internal/store/models.go

@@ -58,11 +58,23 @@ type Settings struct {
 	SSLCertificateID   int    `json:"ssl_certificate_id"`
 	StaleThresholdDays int    `json:"stale_threshold_days"`
 	AllowedVolumePaths string `json:"allowed_volume_paths"` // JSON array of allowed absolute paths
-	WildcardDNS        bool   `json:"wildcard_dns"`
-	DNSProvider        string `json:"dns_provider"`
-	CloudflareAPIToken string `json:"cloudflare_api_token"`
-	CloudflareZoneID   string `json:"cloudflare_zone_id"`
-	UpdatedAt          string `json:"updated_at"`
+	WildcardDNS          bool   `json:"wildcard_dns"`
+	DNSProvider          string `json:"dns_provider"`
+	CloudflareAPIToken   string `json:"cloudflare_api_token"`
+	CloudflareZoneID     string `json:"cloudflare_zone_id"`
+	BackupEnabled        bool   `json:"backup_enabled"`
+	BackupIntervalHours  int    `json:"backup_interval_hours"`
+	BackupRetentionCount int    `json:"backup_retention_count"`
+	UpdatedAt            string `json:"updated_at"`
+}
+
+// Backup represents a backup metadata record.
+type Backup struct {
+	ID         string `json:"id"`
+	Filename   string `json:"filename"`
+	SizeBytes  int64  `json:"size_bytes"`
+	BackupType string `json:"backup_type"` // "manual" or "auto"
+	CreatedAt  string `json:"created_at"`
 }
 
 // DNSRecord tracks a DNS record managed by the application.