feat(tagging): SigLIP concept crops + max-over-bag scoring (#114)

Lift recall on small/local concepts (glasses, cum, stomach-bulge, xray,
lactation) that the whole-image SigLIP vector washes out: the GPU agent now
embeds figure crops with SigLIP too, stored as kind='concept' regions, and the
suggestion rail scores each image as a BAG (whole-image + every concept crop),
taking each head's MAX over the bag. The whole-image vector is always in the
bag, so this can never score lower than before.

Model-agnostic by construction: the server ANNOUNCES the embedding model
(HF name + version) in the lease, so the agent loads whatever the heads were
trained in and stays in lock-step — a model swap is a server setting + a
re-embed migration, never an agent change.

- agent: model-agnostic CropEmbedder (torch/transformers get_image_features,
  fp16 on CUDA, inference-locked); worker branches on job.task — 'ccip' emits
  figure(CCIP)+concept(SigLIP) in one pass, 'siglip' emits concept-only so the
  back-catalogue backfill never churns figure/CCIP regions; torch cu124 +
  transformers in the image.
- server: lease announces embed_model_name/embed_version; score_image is
  max-over-bag (version-filtered region embeddings); enqueue_gpu_backfill
  'siglip' gates on a missing concept region (drains the back-catalogue,
  retries failures, no double-enqueue); daily siglip-backfill beat; UI button;
  /api/ccip/overview reports images_with_concept_siglip.
- v1 scope: suggestion rail only — auto-apply stays whole-image (conservative;
  heads' thresholds were calibrated on whole-image). Bulk-apply bag = follow-up.

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

backend/app/celery_app.py

@@ -126,6 +126,11 @@ def make_celery() -> Celery:
                 "schedule": 3600.0,  # auto-feed new images (+ retry errored) so
                 "args": ("ccip",),   # the queue keeps moving without the button
             },
+            "enqueue-siglip-backfill-daily": {
+                "task": "backend.app.tasks.ml.enqueue_gpu_backfill",
+                "schedule": 86400.0,  # drain the concept-crop back-catalogue +
+                "args": ("siglip",),  # retry failed embeds, no button needed
+            },
             "ccip-auto-apply-daily": {
                 "task": "backend.app.tasks.ml.scheduled_ccip_auto_apply",
                 "schedule": 86400.0,  # no-op unless ccip_auto_apply_enabled