feat(pipeline): render queue, worker service and automated preview generation

One "Queue Export" click now renders the EXR sequence, then rebuilds the shot
headlessly with the studio slate/overlay template to produce the delivery MOV
and review MP4. Implements RenderPipeline2 phases 1-2 plus the preview stage.

Server:
- New models Export, RenderJob, ExportEvent, Machine, WorkerHeartbeat, plus
  Project.deliveryConfig and per-submission slate fields (Export.vfxScope,
  Export.submissionNote, inherited from the shot's previous export).
  Both migrations are purely additive; no existing column is touched.
- lib/render-pipeline: server-enforced state machine, transactional version
  increment with supersede, atomic FOR UPDATE SKIP LOCKED claim gated by
  machine availability windows, and a lease reaper run from instrumentation.ts.
- /api/ext/* endpoints for the panel and workers; session-auth mirrors under
  /api/render and /api/machines for the web UI.
- Pipeline pages: render queue, export detail, machine monitoring, plus an
  Exports tab on shot detail.

RenderWorker (.NET 8 Windows service, new):
- Registration, heartbeat as cancel channel, claim loop, aerender runner with
  progress parsing and stall watchdog, crash recovery and disk-spooled
  reporting that survives server downtime.
- Preview stage: headless AE assembles the preview comp into a throwaway AEP
  with both output modules queued, then a single aerender pass renders them.
  Preview jobs are not claimed while an interactive AE session is open, so an
  artist's project is never taken over.

AE panel: Queue Export with live status polling, urgent flag, retry, and the
VFX Scope / Submission Note fields. Every existing panel action is unchanged.

Preview chaining ships disabled behind SystemConfig preview.enabled.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/render-pipeline/errors.ts

@@ -0,0 +1,26 @@
+/**
+ * Typed service-layer error carrying an HTTP status so both auth fronts
+ * (ext API-key routes and internal session routes) map errors identically.
+ */
+export class PipelineError extends Error {
+  status: number;
+  details?: unknown;
+
+  constructor(status: number, message: string, details?: unknown) {
+    super(message);
+    this.name = "PipelineError";
+    this.status = status;
+    this.details = details;
+  }
+}
+
+export function toErrorResponse(err: unknown): { body: { error: string; details?: unknown }; status: number } {
+  if (err instanceof PipelineError) {
+    return {
+      body: { error: err.message, ...(err.details !== undefined ? { details: err.details } : {}) },
+      status: err.status,
+    };
+  }
+  console.error("[render-pipeline]", err);
+  return { body: { error: "Internal server error" }, status: 500 };
+}