fix: persist plate-clear gate so Auto Off power cycles can't bypass the queue confirmation (#961)

With Auto Off enabled and another job queued, the smart plug cut power when a
  print finished and immediately re-powered the printer because the scheduler
  saw pending items. The printer booted fresh into IDLE and the next job
  auto-dispatched, bypassing the "Clear Plate & Start Next" confirmation.

  Root cause: the plate-clear gate lived only in PrinterManager._plate_cleared
  (in-memory set) and _is_printer_idle treated IDLE as unconditionally idle. On
  power cycle the in-memory flag was lost and the IDLE-on-boot state skipped
  the gate entirely.

  Fix:
  - Replace the in-memory flag with an awaiting_plate_clear column on the
    printers table, rehydrated into the PrinterManager at startup.
  - Set the flag in on_print_complete for completed/failed prints (not user
    cancellations); clear it on ack and on scheduler dispatch.
  - _is_printer_idle now short-circuits to not-idle whenever require_plate_clear
    is on and the flag is set, regardless of the currently reported state —
    so the gate holds through power cycles, Bambuddy restarts, and the printer
    booting back into IDLE.
  - /printers/{id}/clear-plate no longer requires the printer to report
    FINISH/FAILED; it accepts the ack whenever the flag is raised.
  - Frontend widgets (PrinterQueueWidget, Layout, BulkPrinterToolbar) gate on
    the flag rather than reported state.

  Tests: added regression tests for IDLE+awaiting=True (the #961 case) and
  full DB round-trip tests for the persistence layer.

frontend/src/api/client.ts

@@ -283,8 +283,9 @@ export interface PrinterStatus {
   firmware_version: string | null;   // Firmware version from MQTT
   // Developer LAN mode: true = enabled, false = disabled, null = unknown
   developer_mode: boolean | null;
-  // Queue: user has acknowledged plate is cleared for next queued print
-  plate_cleared: boolean;
+  // Queue: printer is awaiting user ack that the build plate was cleared after a
+  // finished/failed print. Persisted across restarts (#961).
+  awaiting_plate_clear: boolean;
   // AMS drying support
   supports_drying: boolean;
 }