feat(backups): scheduled database backups via cron tick

- database_services gains backup_schedule + next_backup_at (goose migration,
  sqlc schema, new queries).
- PUT /databases/:id accepts backup_schedule — validated cron expression,
  empty string disables; invalid returns 400.
- Cron scheduler tick now also folds due databases into the existing
  snapshot pipeline (createBackupProcess).
- Databases page backups section gets a schedule row showing current expr
  and next run.
- resolveBackupConfig reports real next_backup/enabled instead of a fake
  last+24h estimate.
- Verified e2e: '* * * * *' produced three consecutive completed archives
  containing the seeded payload; invalid schedule rejected.

Authored By: TDvorak <[email protected]>

app/frontend/src/lib/api-client.ts

@@ -1295,6 +1295,13 @@ export async function databaseAction(id: string, action: 'start' | 'stop' | 'res
   });
 }
 
+export async function updateDatabaseBackupSchedule(id: string, schedule: string): Promise<void> {
+  await requestJson(`/databases/${encodeURIComponent(id)}`, {
+    method: 'PUT',
+    body: JSON.stringify({ backup_schedule: schedule }),
+  });
+}
+
 export async function createDatabaseBackup(id: string): Promise<void> {
   await requestJson(`/databases/${encodeURIComponent(id)}/backup`, {
     method: 'POST',