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/backups.go

@@ -0,0 +1,106 @@
+package store
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+)
+
+// CreateBackup inserts a new backup metadata record.
+func (s *Store) CreateBackup(b Backup) (Backup, error) {
+	if b.ID == "" {
+		b.ID = uuid.New().String()
+	}
+	b.CreatedAt = Now()
+
+	_, err := s.db.Exec(
+		`INSERT INTO backups (id, filename, size_bytes, backup_type, created_at)
+		 VALUES (?, ?, ?, ?, ?)`,
+		b.ID, b.Filename, b.SizeBytes, b.BackupType, b.CreatedAt,
+	)
+	if err != nil {
+		return Backup{}, fmt.Errorf("insert backup: %w", err)
+	}
+	return b, nil
+}
+
+// GetBackup returns a backup by ID.
+func (s *Store) GetBackup(id string) (Backup, error) {
+	var b Backup
+	err := s.db.QueryRow(
+		`SELECT id, filename, size_bytes, backup_type, created_at
+		 FROM backups WHERE id = ?`, id,
+	).Scan(&b.ID, &b.Filename, &b.SizeBytes, &b.BackupType, &b.CreatedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return Backup{}, fmt.Errorf("backup %s: %w", id, ErrNotFound)
+	}
+	if err != nil {
+		return Backup{}, fmt.Errorf("query backup: %w", err)
+	}
+	return b, nil
+}
+
+// ListBackups returns all backups ordered by creation date descending.
+func (s *Store) ListBackups() ([]Backup, error) {
+	rows, err := s.db.Query(
+		`SELECT id, filename, size_bytes, backup_type, created_at
+		 FROM backups ORDER BY created_at DESC`,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("query backups: %w", err)
+	}
+	defer rows.Close()
+
+	var backups []Backup
+	for rows.Next() {
+		var b Backup
+		if err := rows.Scan(&b.ID, &b.Filename, &b.SizeBytes, &b.BackupType, &b.CreatedAt); err != nil {
+			return nil, fmt.Errorf("scan backup: %w", err)
+		}
+		backups = append(backups, b)
+	}
+	return backups, rows.Err()
+}
+
+// DeleteBackup removes a backup metadata record by ID.
+func (s *Store) DeleteBackup(id string) error {
+	_, err := s.db.Exec(`DELETE FROM backups WHERE id = ?`, id)
+	if err != nil {
+		return fmt.Errorf("delete backup: %w", err)
+	}
+	return nil
+}
+
+// CountBackups returns the total number of backup records.
+func (s *Store) CountBackups() (int, error) {
+	var count int
+	err := s.db.QueryRow(`SELECT COUNT(*) FROM backups`).Scan(&count)
+	if err != nil {
+		return 0, fmt.Errorf("count backups: %w", err)
+	}
+	return count, nil
+}
+
+// GetOldestBackups returns the N oldest backups (for pruning).
+func (s *Store) GetOldestBackups(limit int) ([]Backup, error) {
+	rows, err := s.db.Query(
+		`SELECT id, filename, size_bytes, backup_type, created_at
+		 FROM backups ORDER BY created_at ASC LIMIT ?`, limit,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("query oldest backups: %w", err)
+	}
+	defer rows.Close()
+
+	var backups []Backup
+	for rows.Next() {
+		var b Backup
+		if err := rows.Scan(&b.ID, &b.Filename, &b.SizeBytes, &b.BackupType, &b.CreatedAt); err != nil {
+			return nil, fmt.Errorf("scan backup: %w", err)
+		}
+		backups = append(backups, b)
+	}
+	return backups, rows.Err()
+}