feat(artist): "new since last visit" badge + banner

Per-artist "+N" accent pill on the artists directory and a "N new since
last visit" banner inside ArtistView. Counts new IMAGES (not posts) so
multi-image posts increment correctly.

- alembic 0034: artist_visit (artist_id PK, last_viewed_at NOT NULL).
  Seeds every existing artist with last_viewed_at=NOW() so the badge
  starts at 0 across the board — no noisy "5000 unseen images" on
  first deploy.
- ArtistService.find_or_create autoseeds a visit row alongside new
  artists, so freshly imported content doesn't read as unseen.
- ArtistService.overview reads pre-visit last_viewed_at, counts images
  created since, then atomically UPSERTs last_viewed_at=NOW() via
  postgres ON CONFLICT DO UPDATE (no SELECT-then-INSERT race per
  reference_scalar_one_or_none_duplicates). Returns the pre-update
  count as `unseen_count_at_visit` so the banner has data.
- ArtistDirectoryService.list_artists adds an `unseen_count` aggregate
  to each card via LEFT JOIN artist_visit + conditional COUNT. NULL
  last_viewed_at (artist created before this code shipped) defensively
  counts as "never visited" → all images unseen.
- Frontend: ArtistCard renders an accent pill in the preview-strip
  corner when unseen_count > 0 (capped at 99+); ArtistView shows a
  closable v-alert banner on initial load when
  unseen_count_at_visit > 0, re-arms on slug change.

Single-row-per-artist (no user_id) — rule #47 multi-user ACL is
aspirational; widens to (user_id, artist_id) PK when User lands, per
rule #22.

Scribe plan #597.

frontend/src/views/ArtistView.vue

@@ -20,6 +20,22 @@
       :last-added="store.lastAdded"
     />
     <v-container fluid class="pt-2 pb-4">
+      <!-- "N new since last visit" banner. Visible only on the initial
+           load that triggered the visit-mark; dismissable via close
+           button or by switching tabs. Re-entry only re-shows if more
+           content has arrived (overview returns 0 immediately after a
+           previous visit). -->
+      <v-alert
+        v-if="unseenBanner"
+        type="info" variant="tonal" density="compact"
+        class="mb-3" closable
+        @click:close="unseenBanner = false"
+      >
+        <span class="fc-artist__unseen-msg">
+          <strong>{{ store.overview.unseen_count_at_visit }}</strong>
+          new since last visit
+        </span>
+      </v-alert>
       <v-window v-model="tab">
         <v-window-item value="posts">
           <ArtistPostsTab
@@ -39,7 +55,7 @@
 </template>
 
 <script setup>
-import { computed, watch } from 'vue'
+import { computed, ref, watch } from 'vue'
 import { useRoute } from 'vue-router'
 
 import { useArtistStore } from '../stores/artist.js'
@@ -60,6 +76,10 @@ const { tab, resolve } = useTabQuery(
   () => ((store.postCount ?? 0) > 0 ? 'posts' : 'gallery'),
 )
 
+// One-shot banner — reset on each new artist-slug load so it re-appears
+// when navigating between artists that each have unseen content.
+const unseenBanner = ref(false)
+
 watch(slug, async (s) => {
   if (!s) return
   await store.load(s)
@@ -67,6 +87,7 @@ watch(slug, async (s) => {
     ? `${store.overview.name} — FabledCurator`
     : 'FabledCurator'
   tab.value = resolve()
+  unseenBanner.value = (store.overview?.unseen_count_at_visit || 0) > 0
 }, { immediate: true })
 </script>
 
@@ -74,4 +95,7 @@ watch(slug, async (s) => {
 .fc-artist__loading {
   display: flex; justify-content: center; padding: 64px 0;
 }
+.fc-artist__unseen-msg {
+  font-size: 14px;
+}
 </style>