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/machines/[machineId]/route.ts

@@ -0,0 +1,41 @@
+import { NextRequest, NextResponse } from "next/server";
+import { toErrorResponse, PipelineError } from "@/lib/render-pipeline/errors";
+import { requirePipelineUser, requirePipelineAdmin } from "@/lib/render-pipeline/session-auth";
+import { updateMachine, type MachineAvailability } from "@/lib/render-pipeline/machines";
+
+// ── PATCH /api/machines/{machineId} ──────────────────────────────────────────
+//
+// Body (all optional):
+//   enabled: boolean          — admin kill-switch (admin/producer/supervisor)
+//   availability: {...}|null  — §7.10 schedule config (admin/producer/supervisor)
+//   renderNowHours: number|0  — "Render Now" override (any studio user);
+//                               0 or null clears it
+
+export async function PATCH(
+  req: NextRequest,
+  { params }: { params: Promise<{ machineId: string }> }
+) {
+  try {
+    const { machineId } = await params;
+    let body: { enabled?: boolean; availability?: MachineAvailability | null; renderNowHours?: number | null };
+    try {
+      body = await req.json();
+    } catch {
+      throw new PipelineError(400, "Invalid JSON");
+    }
+
+    // Render Now is available to every studio user (§7.10); enable/disable and
+    // availability windows are admin-level controls.
+    if (body.enabled !== undefined || body.availability !== undefined) {
+      await requirePipelineAdmin();
+    } else {
+      await requirePipelineUser();
+    }
+
+    const result = await updateMachine(machineId, body);
+    return NextResponse.json(result);
+  } catch (err) {
+    const { body, status } = toErrorResponse(err);
+    return NextResponse.json(body, { status });
+  }
+}

app/api/machines/route.ts

@@ -0,0 +1,20 @@
+import { NextResponse } from "next/server";
+import { toErrorResponse } from "@/lib/render-pipeline/errors";
+import { requirePipelineUser } from "@/lib/render-pipeline/session-auth";
+import { listMachines } from "@/lib/render-pipeline/machines";
+
+// ── GET /api/machines (E21) ──────────────────────────────────────────────────
+//
+// Machine list for the Machine Monitoring page: derived ONLINE/OFFLINE status,
+// latest heartbeat stats, current job.
+
+export async function GET() {
+  try {
+    await requirePipelineUser();
+    const machines = await listMachines();
+    return NextResponse.json({ machines });
+  } catch (err) {
+    const { body, status } = toErrorResponse(err);
+    return NextResponse.json(body, { status });
+  }
+}