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

alembic/versions/0069_default_siglip2.py

@@ -0,0 +1,51 @@
+"""default the embedder to SigLIP 2 — for FRESH installs only (#1203)
+
+Make SigLIP 2 (so400m, 512px; a 1152-d drop-in) the default embedder. New
+installs start on it. An EXISTING library is NOT touched: flipping its stored
+embedder version would mark every embedding stale (the scorer is version-gated)
+and kill suggestions until a full re-embed+retrain — so an existing instance
+switches deliberately via Settings → GPU agent → Embedding model → Re-embed →
+Retrain. We detect "fresh" by the absence of any embedded image.
+
+Revision ID: 0069
+Revises: 0068
+Create Date: 2026-06-30
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0069"
+down_revision: Union[str, None] = "0068"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+_NEW_NAME = "google/siglip2-so400m-patch16-512"
+_NEW_VERSION = "siglip2-so400m-patch16-512"
+_OLD_NAME = "google/siglip-so400m-patch14-384"
+_OLD_VERSION = "siglip-so400m-patch14-384"
+
+
+def upgrade() -> None:
+    # Fresh install (nothing embedded yet) → adopt SigLIP 2.
+    op.execute(
+        f"""
+        UPDATE ml_settings SET
+            embedder_model_name = '{_NEW_NAME}',
+            embedder_model_version = '{_NEW_VERSION}'
+        WHERE NOT EXISTS (
+            SELECT 1 FROM image_record WHERE siglip_embedding IS NOT NULL
+        )
+        """
+    )
+    op.alter_column("ml_settings", "embedder_model_name", server_default=_NEW_NAME)
+    op.alter_column(
+        "ml_settings", "embedder_model_version", server_default=_NEW_VERSION
+    )
+
+
+def downgrade() -> None:
+    op.alter_column("ml_settings", "embedder_model_name", server_default=_OLD_NAME)
+    op.alter_column(
+        "ml_settings", "embedder_model_version", server_default=_OLD_VERSION
+    )