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>

app/api/ext/workers/[machineId]/heartbeat/route.ts

@@ -0,0 +1,33 @@
+import { NextRequest, NextResponse } from "next/server";
+import { isExtAuthorized } from "@/lib/render-pipeline/ext-auth";
+import { toErrorResponse } from "@/lib/render-pipeline/errors";
+import { recordHeartbeat } from "@/lib/render-pipeline/machines";
+
+// ── POST /api/ext/workers/{machineId}/heartbeat (E7) ─────────────────────────
+//
+// Heartbeat + the server→worker command channel: cancellation piggybacks on
+// the response (`commands: [{ type: "CANCEL_JOB", jobId }]`) so no server→
+// worker connection is ever needed.
+
+export async function POST(
+  req: NextRequest,
+  { params }: { params: Promise<{ machineId: string }> }
+) {
+  if (!isExtAuthorized(req)) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+  const { machineId } = await params;
+  let body: { cpuPercent?: number; memPercent?: number; diskFreeGb?: number; currentJobId?: string | null } = {};
+  try {
+    body = await req.json();
+  } catch {
+    // heartbeat with empty body is fine
+  }
+  try {
+    const result = await recordHeartbeat(machineId, body);
+    return NextResponse.json(result);
+  } catch (err) {
+    const { body: errBody, status } = toErrorResponse(err);
+    return NextResponse.json(errBody, { status });
+  }
+}