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]>

docs/api/openapi.yaml

@@ -4778,6 +4778,13 @@ components:
         region:
           type: string
           description: Provisioning region
+        backup_schedule:
+          type: string
+          description: Cron expression for automatic backups, empty when unset
+        next_backup_at:
+          type: string
+          format: date-time
+          description: Next scheduled backup run
         metrics:
           $ref: '#/components/schemas/DatabaseMetrics'
         backups:
@@ -4826,6 +4833,10 @@ components:
           type: string
           enum: [hobby, starter, standard, business]
           description: Updated database plan
+        backup_schedule:
+          type: string
+          nullable: true
+          description: Cron expression for automatic backups; empty string clears the schedule
 
     DatabaseActionRequest:
       type: object