fix(audit-g5): centroid version DB-as-truth + modal as overlay

Closes the last two findings from the 2026-06-02 audit (G5.1 + G5.4).

G5.1 — Centroid version no longer drifts:

CentroidService now reads MLSettings.embedder_model_version (the DB
row tag_and_embed already writes from) for both the centroid model-
version stamp and the drift-detection comparison. Previously the
centroid sites imported MODEL_VERSION from env, so the version stamped
on centroids could disagree with the version stamped on the embeddings
they were built from. By construction those now match, so list_drifted
won't silently miss the env-vs-DB drift case.

embedder.py keeps MODEL_VERSION as an env-driven constant for the
actual model loader — that's a different concern (which weights are
loaded) from the version-stamp that gets persisted alongside data.

G5.4 — Modal is a Pinia-only overlay:

The previous URL↔modal sync in GalleryView and ArtistGalleryTab
leaked the modal across route changes (RouterLink to /artist/<slug>
left the modal mounted on top of the new route) and re-opened it
on history back/forward with stale ?image=N entries.

Now: openImage() just calls modal.open(id) — no URL push.
GalleryView's dead closeImage helper is deleted. A route.name
watcher in App.vue closes the modal whenever the route changes,
which auto-fixes RouterLink-in-modal and back/forward.

Backward-compat: ?image=N is still honored on initial mount as a
one-shot deep-link opener, then router.replace strips the query so
the URL doesn't re-trigger and no extra history entry is added.
Existing bookmarks / shared URLs keep working; new opens stay
Pinia-only.

backend/app/services/ml/centroids.py

@@ -19,7 +19,6 @@ from ...models import (
     TagReferenceEmbedding,
 )
 from ...models.tag import image_tag
-from .embedder import MODEL_VERSION as SIGLIP_VERSION
 
 ELIGIBLE_KINDS = {
     TagKind.character,
@@ -46,6 +45,21 @@ class CentroidService:
             )
         ).scalar_one()
 
+    async def _model_version(self) -> str:
+        """Audit 2026-06-02: SigLIP model-version stamp comes from the
+        DB row, not the env constant. tag_and_embed (tasks/ml.py:110)
+        already reads from MLSettings.embedder_model_version, so by
+        sourcing centroid stamps + drift checks from the same row, we
+        eliminate the silent-drift case the audit flagged. env
+        SIGLIP_MODEL_VERSION still drives which model embedder.py
+        loads at runtime; the version stamp is purely the operator-
+        controlled identifier."""
+        return (
+            await self.session.execute(
+                select(MLSettings.embedder_model_version).where(MLSettings.id == 1)
+            )
+        ).scalar_one()
+
     async def recompute_for_tag(self, tag_id: int) -> bool:
         """Recompute one tag's centroid. Returns True if a centroid was
         written, False if skipped (ineligible kind or too few members)."""
@@ -69,19 +83,20 @@ class CentroidService:
             return False
 
         centroid = np.mean(np.stack(embeddings), axis=0).astype(np.float32)
+        model_version = await self._model_version()
 
         stmt = insert(TagReferenceEmbedding).values(
             tag_id=tag_id,
             embedding=centroid.tolist(),
             reference_count=len(embeddings),
-            model_version=SIGLIP_VERSION,
+            model_version=model_version,
         )
         stmt = stmt.on_conflict_do_update(
             index_elements=["tag_id"],
             set_={
                 "embedding": centroid.tolist(),
                 "reference_count": len(embeddings),
-                "model_version": SIGLIP_VERSION,
+                "model_version": model_version,
                 "updated_at": func.now(),
             },
         )
@@ -92,6 +107,7 @@ class CentroidService:
         """Tag ids whose centroid is stale: member count != reference_count,
         OR no centroid row, OR centroid built on a different SigLIP version.
         Only considers eligible-kind tags with embeddings present."""
+        current_model_version = await self._model_version()
         member_counts = (
             select(
                 image_tag.c.tag_id.label("tag_id"),
@@ -116,7 +132,7 @@ class CentroidService:
                     TagReferenceEmbedding.reference_count
                     != member_counts.c.members
                 )
-                | (TagReferenceEmbedding.model_version != SIGLIP_VERSION)
+                | (TagReferenceEmbedding.model_version != current_model_version)
             )
         )
         return list((await self.session.execute(stmt)).scalars().all())