feat(smart-plugs): auto-off after AMS drying completes (#1349)

Reporter Kyobinoyo asked for the equivalent of the existing
  print-finish auto-off but triggered when AMS drying ends.

  Two new SmartPlug columns: auto_off_after_drying (default false),
  off_delay_after_drying_minutes (default 10 — AMS chamber is hot
  post-cycle so longer cooldown than the print-finish default of 5).
  SQLite + Postgres migrations both idempotent.

  Trigger lives in BambuMQTTClient — per-AMS _previous_dry_times
  tracks the dry_time > 0 → 0 falling edge and fires a new
  on_drying_complete(ams_id) callback. Plumbed through
  PrinterManager.set_drying_complete_callback to
  SmartPlugManager.on_drying_complete(printer_id, db), which walks
  linked plugs and respects the per-plug toggle. Catches queue,
  ambient and manual drying identically because it observes firmware
  state, not scheduler intent.

  Frontend: single "Auto Off After Drying" toggle + delay input on
  the smart plug card, next to the existing print-finish auto-off
  section.

  Per-AMS plug routing (separate plug for AMS only, per-AMS targeting
  on dual-AMS printers) deferred — Bambuddy's plug model is
  plug→printer, so the trigger fires whenever any AMS on the linked
  printer finishes a cycle.

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