feat(ml): operator model swap — GPU re-embed + embedder as a setting (#1190)

Make the SigLIP embedder an operator choice (drop-in to SigLIP 2:
google/siglip2-so400m-patch16-512 is a verified 1152-d model at 512px → no
schema change, better small-cue fidelity). A swap = set model + re-embed +
retrain, all operator-driven; the GPU agent does the re-embed so it's fast.

- settings: embedder_model_name is now a setting (migration 0065) alongside the
  existing embedder_model_version; both editable + validated (non-empty) in the
  ml admin API. The server embedder loads by HF name (AutoImageProcessor/Model,
  model-agnostic), preferring the pre-downloaded local dir for the default so
  existing deploys don't re-download; rebuilds on a name change.
- agent: new 'embed' job = whole-image SigLIP embedding (mean-pool video frames)
  under the lease-announced model → POST /jobs/submit_embedding writes
  image_record.siglip_embedding + siglip_model_version. The lease now announces
  the model FROM THE SETTING (not a constant).
- re-embed routing: enqueue_gpu_backfill('embed') selects unembedded + stale-
  version images; 'siglip' now re-embeds concept crops whose version != current
  (so a swap re-triggers crops, not just the never-embedded back-catalogue). The
  CPU ml-worker backfill no longer re-embeds on a version mismatch (it can't
  churn the library at 512px) — the GPU agent owns version re-embeds. Daily
  'embed' + 'siglip' beats self-heal.
- scoring: score_image only bags embeddings in the CURRENT model's space (whole-
  image gated by siglip_model_version, concept regions by embedding_version) so a
  mid-swap stale vector isn't scored by new-space heads; legacy NULL = current.
- UI: GpuAgentCard "Embedding model (advanced)" — edit name/version, Save, and
  "Re-embed library (GPU)" (queues embed + siglip); points at SigLIP 2.

Tests: lease announces model + submit_embedding round-trip; enqueue 'embed'
selects stale/unembedded; stale-version excluded from scoring; embedder model
settable + empty rejected; siglip gate updated to current-version concept.

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

backend/app/services/ml/heads.py

@@ -308,25 +308,36 @@ async def score_image(
     import numpy as np
 
     img = await session.get(ImageRecord, image_id)
-    if img is None or img.siglip_embedding is None:
+    if img is None:
         return []
     settings = await _settings_async(session)
-    heads = await _current_heads(session, settings.embedder_model_version)
+    cur_version = settings.embedder_model_version
+    heads = await _current_heads(session, cur_version)
     if heads["W"] is None:
         return []
 
-    bag = [np.asarray(img.siglip_embedding, dtype=np.float32)]
+    # Only embeddings in the CURRENT model's space enter the bag. Mid model-swap
+    # (#1190), an image still carrying the OLD-version whole-image vector is
+    # skipped rather than scored by heads trained in a different space; a legacy
+    # NULL version is treated as current (those predate per-row stamping).
+    bag = []
+    if img.siglip_embedding is not None and img.siglip_model_version in (
+        cur_version, None,
+    ):
+        bag.append(np.asarray(img.siglip_embedding, dtype=np.float32))
     region_vecs = (
         await session.execute(
             select(ImageRegion.siglip_embedding)
             .where(ImageRegion.image_record_id == image_id)
             .where(ImageRegion.siglip_embedding.is_not(None))
-            .where(ImageRegion.embedding_version == settings.embedder_model_version)
+            .where(ImageRegion.embedding_version == cur_version)
         )
     ).all()
     for (vec,) in region_vecs:
         if vec is not None:
             bag.append(np.asarray(vec, dtype=np.float32))
+    if not bag:
+        return []
 
     X = np.vstack(bag)                                  # (B, D)
     norms = np.linalg.norm(X, axis=1, keepdims=True)