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/tasks/ml.py

@@ -742,24 +742,43 @@ def scheduled_apply_head_tags() -> str:
 
 @celery.task(name="backend.app.tasks.ml.enqueue_gpu_backfill")
 def enqueue_gpu_backfill(task_name: str) -> int:
-    """Enqueue a gpu_job for every image that doesn't already have one for
-    `task_name` (one INSERT…SELECT, so it scales to a full library). The desktop
-    agent drains the queue over HTTP. Returns the number enqueued."""
+    """Enqueue a gpu_job for every image that still needs `task_name` (one
+    INSERT…SELECT, so it scales to a full library). The desktop agent drains the
+    queue over HTTP. Returns the number enqueued.
+
+    'siglip' gates on the RESULT (no concept region yet) rather than on a prior
+    job, so it picks up the back-catalogue of images that were CCIP-embedded
+    before concept crops existed, and retries images whose concept embed failed —
+    without re-touching their figure/CCIP regions."""
     from sqlalchemy import exists, insert, literal
     from sqlalchemy import select as sa_select
 
-    from ..models import GpuJob, ImageRecord
+    from ..models import GpuJob, ImageRecord, ImageRegion
 
     SessionLocal = _sync_session_factory()
     with SessionLocal() as session:
-        already = exists().where(
-            GpuJob.image_record_id == ImageRecord.id,
-            GpuJob.task == task_name,
-            GpuJob.status.in_(["pending", "leased", "done"]),
-        )
-        sel = sa_select(
-            ImageRecord.id, literal(task_name), literal("pending")
-        ).where(~already)
+        if task_name == "siglip":
+            has_concept = exists().where(
+                ImageRegion.image_record_id == ImageRecord.id,
+                ImageRegion.kind == "concept",
+            )
+            queued = exists().where(
+                GpuJob.image_record_id == ImageRecord.id,
+                GpuJob.task == "siglip",
+                GpuJob.status.in_(["pending", "leased"]),
+            )
+            sel = sa_select(
+                ImageRecord.id, literal("siglip"), literal("pending")
+            ).where(~has_concept).where(~queued)
+        else:
+            already = exists().where(
+                GpuJob.image_record_id == ImageRecord.id,
+                GpuJob.task == task_name,
+                GpuJob.status.in_(["pending", "leased", "done"]),
+            )
+            sel = sa_select(
+                ImageRecord.id, literal(task_name), literal("pending")
+            ).where(~already)
         # RETURNING + count: result.rowcount is unreliable for INSERT…SELECT.
         rows = session.execute(
             insert(GpuJob)