fix(agent): prompt stop + lazy curator polling + build marker; add agent to CI

Addresses operator reports: Stop never finishes, the agent polls curator
constantly, and stale-cached pages get mistaken for a failed deploy.

- Stop is prompt: flip _running BEFORE any lock so /status + worker loops see
  "stopped" immediately, and add a stop/shrink checkpoint in _process (after
  decode, before the expensive detect+embed) that releases the job and bails —
  so a Stop doesn't wait out heavy GPU work.
- Lazy curator polling: the queue snapshot is fetched only while a browser is
  actually watching (a /status hit within UI_IDLE_GRACE) and on a 5s cadence,
  not a constant background loop. The work loop's own lease/submit is curator's
  only visitor otherwise — nothing polls just to poll.
- Build marker: VERSION is embedded in the page and reported on /status; the UI
  shows a "reload" banner when they differ, so a browser-cached page can't be
  mistaken for "the new image didn't deploy" (complements the no-store header).

CI: the lint lane now also `ruff check`s agent/ and compileall-parses it, so the
GPU agent is linted + syntax-checked before its image builds (build.yml only
`docker build`s it). Fixed the agent's pre-existing UP037/B905 so it passes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

.forgejo/workflows/ci.yml

@@ -27,7 +27,14 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - name: Ruff lint
-        run: ruff check backend/ tests/ alembic/
+        # agent/ included so the GPU-agent is linted before its image is built
+        # (build.yml only `docker build`s it — this is where it gets checked).
+        run: ruff check backend/ tests/ alembic/ agent/
+      - name: Agent syntax check
+        # The agent's runtime deps (torch/transformers/ultralytics) aren't in the
+        # CI image, so we can't import it — but compileall parses every module,
+        # catching syntax errors before the image build.
+        run: python -m compileall -q agent/fc_agent
 
   backend-lint-and-test:
     runs-on: python-ci