feat: model-on-demand lifecycle — retry on 503, live status pill, warming indicator

- whisper.ts: add getModelStatus(); fix submitJob() to retry on 503 using
  Retry-After header instead of throwing; optional onModelWaiting callback
  lets the pipeline surface model state to the UI during the wait
- pipeline.ts: pass onModelWaiting callback → emits model_warming SSE event
  so the job detail page can show 'Warming up model…' while waiting
- types.ts: add ModelStateTag union and ModelStatus interface
- api/model/status: GET route proxies whisper /model/status (falls back to
  {state:'unloaded'} if whisper unreachable)
- api/model/events: GET route relays whisper SSE stream to the browser;
  AbortController tied to request.signal cleans up on disconnect
- layout.svelte: status pill is now live — initial fetch + EventSource on
  /api/model/events; dot colour + label reflect real model state with a
  pulsing animation while loading or waiting_for_gpu
- jobs/[id]/+page.svelte: handle model_warming event type → show a yellow
  'Warming up model…' sub-label with spinner inside the progress card
- whisper.test.ts: update submitJob mocks to status:202 to match real API

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/lib/types.ts

@@ -1,5 +1,17 @@
 export type AudioMode = 'auto' | 'standard' | 'aggressive' | 'none';
 
+export type ModelStateTag = 'unloaded' | 'loading' | 'waiting_for_gpu' | 'ready';
+
+export interface ModelStatus {
+	state: ModelStateTag;
+	loaded_at?: string;
+	vram_needed_mb?: number;
+	vram_free_mb?: number;
+	retry_in_secs?: number;
+	vram_used_mb?: number;
+	vram_total_mb?: number;
+}
+
 export type JobStatus = 'pending' | 'downloading' | 'preparing' | 'transcribing' | 'processing' | 'done' | 'failed' | 'cancelled';
 
 export interface Segment {