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/lib/api/settings.ts

@@ -215,3 +215,19 @@ export interface LogConfig {
   enabled: boolean;
   level: "error" | "warn" | "info" | "debug" | "trace";
 }
+
+export interface BackupEntry {
+  filename: string;
+  sizeBytes: number;
+  createdAt: string;
+}
+
+export const backupsApi = {
+  async listDbBackups(): Promise<BackupEntry[]> {
+    return await invoke("list_db_backups");
+  },
+
+  async restoreDbBackup(filename: string): Promise<string> {
+    return await invoke("restore_db_backup", { filename });
+  },
+};