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.

frontend/src/views/GalleryView.vue

@@ -47,9 +47,20 @@ onMounted(async () => {
   else if (!isNaN(tagId)) store.setTagFilter(tagId)
   await store.loadInitial()
   await store.loadTimeline()
-  // Open modal if URL has ?image=N
+  // Audit 2026-06-02: modal is now a Pinia-only overlay (see
+  // G5.4 design). The previous URL↔modal sync leaked the modal
+  // across route changes and re-opened it on history back/forward.
+  // We still honor `?image=N` as a one-shot deep-link opener so
+  // existing bookmarks / shared URLs keep working; we strip the
+  // query immediately via router.replace so the URL doesn't
+  // re-trigger and no history entry is added.
   const initial = parseInt(route.query.image, 10)
-  if (!isNaN(initial)) modal.open(initial)
+  if (!isNaN(initial)) {
+    modal.open(initial)
+    const q = { ...route.query }
+    delete q.image
+    router.replace({ query: q })
+  }
 })
 
 watch(() => route.query.tag_id, (q) => {
@@ -64,19 +75,8 @@ watch(() => route.query.post_id, (q) => {
   store.setPostFilter(isNaN(postId) ? null : postId)
 })
 
-watch(() => route.query.image, (q) => {
-  const id = parseInt(q, 10)
-  if (!isNaN(id) && id !== modal.currentImageId) modal.open(id)
-  else if (isNaN(id) && modal.currentImageId !== null) modal.close()
-})
-
 function openImage(id) {
-  router.push({ query: { ...route.query, image: id } })
-}
-function closeImage() {
-  const q = { ...route.query }
-  delete q.image
-  router.push({ query: q })
+  modal.open(id)
 }
 </script>