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/celery_app.py

@@ -29,6 +29,7 @@ def make_celery() -> Celery:
             "backend.app.tasks.thumbnail",
             "backend.app.tasks.maintenance",
             "backend.app.tasks.ml",
+            "backend.app.tasks.gpu_queue",
             "backend.app.tasks.download",
             "backend.app.tasks.external",
             "backend.app.tasks.backup",
@@ -41,6 +42,11 @@ def make_celery() -> Celery:
         task_routes={
             "backend.app.tasks.import_file.*": {"queue": "import"},
             "backend.app.tasks.ml.*": {"queue": "ml"},
+            # GPU-queue coordination (backfill enqueues, orphan recovery,
+            # reprocess) is pure DB work — it rides the maintenance quick lane
+            # so the GPU agent pipeline works even on stacks that drop the
+            # (now-optional, B3) ml-worker container entirely.
+            "backend.app.tasks.gpu_queue.*": {"queue": "maintenance"},
             "backend.app.tasks.thumbnail.*": {"queue": "thumbnail"},
             "backend.app.tasks.download.*": {"queue": "download"},
             # External file-host fetches are downloads — same lane (they can run
@@ -106,7 +112,7 @@ def make_celery() -> Celery:
                 "schedule": 86400.0,  # no-op unless head_auto_apply_enabled
             },
             "recover-orphaned-gpu-jobs": {
-                "task": "backend.app.tasks.ml.recover_orphaned_gpu_jobs",
+                "task": "backend.app.tasks.gpu_queue.recover_orphaned_gpu_jobs",
                 "schedule": 60.0,  # quick pickup of work a dead agent orphaned
             },
             "triage-gpu-errors": {
@@ -114,17 +120,17 @@ def make_celery() -> Celery:
                 "schedule": 900.0,  # probe errored jobs' files → defect/file_ok
             },
             "enqueue-ccip-backfill-hourly": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 3600.0,  # auto-feed NEW images; errored are
                 "args": ("ccip",),   # tombstoned — retry is the button only
             },
             "enqueue-siglip-backfill-daily": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 86400.0,  # drain the concept-crop back-catalogue
                 "args": ("siglip",),  # (errored are tombstoned, not retried)
             },
             "enqueue-embed-backfill-daily": {
-                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "task": "backend.app.tasks.gpu_queue.enqueue_gpu_backfill",
                 "schedule": 86400.0,  # whole-image re-embed under the current
                 "args": ("embed",),   # model (an operator swap) drains via agent
             },