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

frontend/src/stores/ml.js

@@ -22,8 +22,14 @@ export const useMLStore = defineStore('ml', () => {
     await api.post('/api/ml/backfill')
   }
 
+  // Server-authoritative list of supported embedders for the model dropdown.
+  async function embedderModels() {
+    const r = await api.get('/api/ml/embedder-models')
+    return r.models || []
+  }
+
   return {
     settings, loading, error,
-    loadSettings, patchSettings, triggerBackfill
+    loadSettings, patchSettings, triggerBackfill, embedderModels
   }
 })