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/store.go

@@ -95,6 +95,10 @@ func (s *Store) runMigrations() error {
 		`ALTER TABLE settings ADD COLUMN dns_provider TEXT NOT NULL DEFAULT ''`,
 		`ALTER TABLE settings ADD COLUMN cloudflare_api_token TEXT NOT NULL DEFAULT ''`,
 		`ALTER TABLE settings ADD COLUMN cloudflare_zone_id TEXT NOT NULL DEFAULT ''`,
+		// Add backup management fields to settings (2026-04-02).
+		`ALTER TABLE settings ADD COLUMN backup_enabled INTEGER NOT NULL DEFAULT 0`,
+		`ALTER TABLE settings ADD COLUMN backup_interval_hours INTEGER NOT NULL DEFAULT 24`,
+		`ALTER TABLE settings ADD COLUMN backup_retention_count INTEGER NOT NULL DEFAULT 10`,
 	}
 
 	for _, m := range migrations {
@@ -313,6 +317,14 @@ CREATE TABLE IF NOT EXISTS dns_records (
 	created_at         TEXT NOT NULL DEFAULT (datetime('now')),
 	updated_at         TEXT NOT NULL DEFAULT (datetime('now'))
 );
+
+CREATE TABLE IF NOT EXISTS backups (
+	id          TEXT PRIMARY KEY,
+	filename    TEXT NOT NULL UNIQUE,
+	size_bytes  INTEGER NOT NULL DEFAULT 0,
+	backup_type TEXT NOT NULL DEFAULT 'manual',
+	created_at  TEXT NOT NULL DEFAULT (datetime('now'))
+);
 `
 
 // Now returns the current time formatted for SQLite storage.