feat(backup): add pre-migration backup, periodic backup, backfill warning, and backup management UI

Four improvements to the database backup mechanism:

1. Auto backup before schema migration - creates a snapshot when
   upgrading from an older database version, providing a safety net
   beyond the existing SAVEPOINT rollback mechanism.

2. Periodic startup backup - checks on app launch whether the latest
   backup is older than 24 hours and creates a new one if needed,
   ensuring all users have recent backups regardless of usage patterns.

3. Backfill failure notification - switch now returns SwitchResult with
   warnings instead of silently ignoring backfill errors, so users are
   informed when their manual config changes may not have been saved.

4. Backup management UI - new BackupListSection in Settings > Data
   Management showing all backup snapshots with restore capability,
   including a confirmation dialog and automatic safety backup before
   restore.

src-tauri/src/commands/import_export.rs

@@ -8,6 +8,8 @@ use tauri_plugin_dialog::DialogExt;
 use crate::commands::sync_support::{
     post_sync_warning_from_result, run_post_import_sync, success_payload_with_warning,
 };
+use crate::database::backup::BackupEntry;
+use crate::database::Database;
 use crate::error::AppError;
 use crate::services::provider::ProviderService;
 use crate::store::AppState;
@@ -118,3 +120,24 @@ pub async fn open_zip_file_dialog<R: tauri::Runtime>(
 
     Ok(result.map(|p| p.to_string()))
 }
+
+// ─── Database backup management ─────────────────────────────
+
+/// List all database backup files
+#[tauri::command]
+pub fn list_db_backups() -> Result<Vec<BackupEntry>, String> {
+    Database::list_backups().map_err(|e| e.to_string())
+}
+
+/// Restore database from a backup file
+#[tauri::command]
+pub async fn restore_db_backup(
+    state: State<'_, AppState>,
+    filename: String,
+) -> Result<String, String> {
+    let db = state.db.clone();
+    tauri::async_runtime::spawn_blocking(move || db.restore_from_backup(&filename))
+        .await
+        .map_err(|e| format!("Restore failed: {e}"))?
+        .map_err(|e: AppError| e.to_string())
+}