feat(ml): default to SigLIP 2 (new installs) + model dropdown, no free-text (#1203)

- Migration 0069: new installs default to SigLIP 2 (so400m, 512px, 1152-d drop-in)
  — UPDATE applies ONLY where no image is embedded yet (fresh install), so an
  existing library is NOT silently invalidated; it switches deliberately via the
  dropdown → Re-embed → Retrain. Column server_defaults moved to SigLIP 2.
- GET /api/ml/embedder-models: server-authoritative supported list (SigLIP 2 512
  recommended / 384 faster / SigLIP 1 384 original) so the UI never free-types.
- GpuAgentCard: the two name/version text fields → a single model dropdown;
  Save sets name+version from the picked option (the current model is always
  selectable even if off-list).
- embedder.py DEFAULT_MODEL_NAME unchanged (stays the baked local-dir SigLIP 1)
  to avoid a local-dir/weights mismatch; SigLIP 2 loads by HF name, cached on the
  ml-worker's persistent HF_HOME.

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

backend/app/api/ml_admin.py

@@ -23,6 +23,36 @@ _EDITABLE = (
 )
 
 
+# Supported embedders for the Settings dropdown — all 1152-d so a swap is a
+# drop-in (re-embed + retrain, no schema change). Server-authoritative so the UI
+# never free-types a model name.
+SUPPORTED_EMBEDDERS = (
+    {
+        "name": "google/siglip2-so400m-patch16-512",
+        "version": "siglip2-so400m-patch16-512",
+        "label": "SigLIP 2 · so400m · 512px (recommended)",
+        "dim": 1152,
+    },
+    {
+        "name": "google/siglip2-so400m-patch16-384",
+        "version": "siglip2-so400m-patch16-384",
+        "label": "SigLIP 2 · so400m · 384px (faster)",
+        "dim": 1152,
+    },
+    {
+        "name": "google/siglip-so400m-patch14-384",
+        "version": "siglip-so400m-patch14-384",
+        "label": "SigLIP 1 · so400m · 384px (original)",
+        "dim": 1152,
+    },
+)
+
+
+@ml_admin_bp.route("/embedder-models", methods=["GET"])
+async def embedder_models():
+    return jsonify({"models": list(SUPPORTED_EMBEDDERS)})
+
+
 @ml_admin_bp.route("/settings", methods=["GET"])
 async def get_settings():
     from sqlalchemy import select