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/render/jobs/[jobId]/artifact-presign/route.ts

@@ -0,0 +1,57 @@
+import { NextRequest, NextResponse } from "next/server";
+import { randomUUID } from "crypto";
+import { isExtAuthorized } from "@/lib/render-pipeline/ext-auth";
+import { toErrorResponse, PipelineError } from "@/lib/render-pipeline/errors";
+import { db } from "@/lib/db";
+import { generateHetznerPresignedUploadUrl, sanitizeFileName } from "@/lib/storage";
+
+// ── POST /api/ext/render/jobs/{jobId}/artifact-presign (E20) ─────────────────
+//
+// Presigned upload URL for worker artifacts. Kinds map to the existing object
+// storage folder layout: preview → videos/, thumbnail → image/,
+// metadata/log → renders/.
+
+const KIND_FOLDERS: Record<string, string> = {
+  preview: "videos",
+  thumbnail: "image",
+  metadata: "renders",
+  log: "renders",
+};
+
+export async function POST(
+  req: NextRequest,
+  { params }: { params: Promise<{ jobId: string }> }
+) {
+  if (!isExtAuthorized(req)) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+  const { jobId } = await params;
+  let body: { machineId?: string; kind?: string; fileName?: string; contentType?: string };
+  try {
+    body = await req.json();
+  } catch {
+    return NextResponse.json({ error: "Invalid JSON" }, { status: 400 });
+  }
+  try {
+    if (!body.machineId) throw new PipelineError(400, "machineId is required");
+    if (!body.fileName) throw new PipelineError(400, "fileName is required");
+    if (!body.contentType) throw new PipelineError(400, "contentType is required");
+    const folder = KIND_FOLDERS[body.kind ?? ""];
+    if (!folder) {
+      throw new PipelineError(422, `kind must be one of: ${Object.keys(KIND_FOLDERS).join(", ")}`);
+    }
+
+    const job = await db.renderJob.findUnique({ where: { id: jobId }, select: { machineId: true } });
+    if (!job) throw new PipelineError(404, "Render job not found");
+    if (job.machineId !== body.machineId) {
+      throw new PipelineError(409, "Job is not held by this machine (lease reassigned?)");
+    }
+
+    const key = `${folder}/${randomUUID()}-${sanitizeFileName(body.fileName)}`;
+    const presignedUrl = await generateHetznerPresignedUploadUrl(key, body.contentType);
+    return NextResponse.json({ presignedUrl, key, url: `/api/files/${key}` });
+  } catch (err) {
+    const { body: errBody, status } = toErrorResponse(err);
+    return NextResponse.json(errBody, { status });
+  }
+}