feat(b3): ml-worker becomes optional — embed-only role, decoupled GPU coordination, cpu-embed switch

The ml-worker's ONLY processing role is now the CPU whole-image embed fallback
(tag_and_embed renamed embed_image — Camie tagging was retired #1189 and the
name kept implying otherwise; videos were already handled agent-style: frame
sampling + mean-pool). Detection/cropping/CCIP stay GPU-agent-only, and their
completion is judged per-pipeline: ccip by gpu_job rows, siglip by concept
regions at the current model version — never by image_record.siglip_embedding.
A CPU embed therefore can NEVER close crop work for the agent (regression test
pins this; only the whole-image 'embed' job, the same artifact, is satisfied).

Making removal actually safe (operator will drop the container):
- GPU-queue coordination (enqueue_gpu_backfill, recover_orphaned_gpu_jobs,
  reprocess_gpu_jobs) moved verbatim to tasks/gpu_queue.py on the maintenance
  quick lane — it lived on the 'ml' queue only by module colocation, which made
  the ml-worker a hard dependency of the whole agent pipeline.
- New ml_settings.cpu_embed_enabled (migration 0074, default ON so agent-less
  installs keep working): OFF stops the four import hooks queueing embed work
  nothing will consume and no-ops the manual backfill; switch lives on the
  renamed 'CPU embedding backfill' card.
- NB heads training / auto-apply still run on the ml image (sklearn) — a stack
  that removes the container gives those up too.

Deploy note: in-flight messages under the old task names are dropped by the
new workers; the 60s orphan sweep + hourly backfill re-fire under the new
names immediately.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_external_worker.py

@@ -125,7 +125,7 @@ def test_refetch_same_link_keeps_canonical_file(db_sync, tmp_path, monkeypatch):
 
     import backend.app.tasks.ml as ml_mod
     import backend.app.tasks.thumbnail as thumb_mod
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda i: None)
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda i: None)
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda i: None)
 
     out = ext.fetch_external_link(link_id)
@@ -234,7 +234,7 @@ def test_downloaded_archive_gets_provenance_and_tagging(db_sync, tmp_path, monke
     tagged, thumbed = [], []
     import backend.app.tasks.ml as ml_mod
     import backend.app.tasks.thumbnail as thumb_mod
-    monkeypatch.setattr(ml_mod.tag_and_embed, "delay", lambda i: tagged.append(i))
+    monkeypatch.setattr(ml_mod.embed_image, "delay", lambda i: tagged.append(i))
     monkeypatch.setattr(thumb_mod.generate_thumbnail, "delay", lambda i: thumbed.append(i))
 
     out = ext.fetch_external_link(link_id)