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

backend/app/services/download_service.py

@@ -326,14 +326,16 @@ class DownloadService:
                 # for hours after a download landed. Lazy import to avoid
                 # circular-import risk between this service and the
                 # tasks/* modules that import it.
-                from ..tasks.ml import tag_and_embed
+                from ..tasks.ml import cpu_embed_enabled, embed_image
                 from ..tasks.thumbnail import generate_thumbnail
+                do_embed = cpu_embed_enabled()
                 ids = list(result.member_image_ids)
                 if result.image_id is not None and result.image_id not in ids:
                     ids.append(result.image_id)
                 for img_id in ids:
                     generate_thumbnail.delay(img_id)
-                    tag_and_embed.delay(img_id)
+                    if do_embed:
+                        embed_image.delay(img_id)
             elif result.status == "attached":
                 # Non-media or extracted archive captured as PostAttachment
                 # (FC-2d-iii). The canonical copy lives in the attachments