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/components/discovery/ArtistCard.vue

@@ -8,6 +8,15 @@
       <div v-if="card.preview_thumbnails.length === 0" class="fc-artistcard__noimg">
         No preview
       </div>
+      <!-- Accent pill in the corner when this artist has content imported
+           since the operator last opened their detail view. Caps at 99+
+           to keep the layout compact; the actual count appears in the
+           banner inside ArtistView. -->
+      <span
+        v-if="(card.unseen_count || 0) > 0"
+        class="fc-artistcard__unseen"
+        :aria-label="`${card.unseen_count} new since last visit`"
+      >+{{ card.unseen_count > 99 ? '99+' : card.unseen_count }}</span>
     </div>
     <v-card-text class="fc-artistcard__body">
       <div class="fc-artistcard__name">{{ card.name }}</div>
@@ -37,6 +46,7 @@ function onCardClick() {
 <style scoped>
 .fc-artistcard { cursor: pointer; }
 .fc-artistcard__previews {
+  position: relative;
   display: grid; grid-template-columns: repeat(3, 1fr);
   gap: 2px; aspect-ratio: 3 / 1;
   /* Explicit floor + ceiling so tall source images can't escape the
@@ -45,6 +55,19 @@ function onCardClick() {
   overflow: hidden;
   background: rgb(var(--v-theme-surface-light));
 }
+.fc-artistcard__unseen {
+  position: absolute;
+  top: 6px; right: 6px;
+  display: inline-flex; align-items: center;
+  padding: 2px 8px;
+  font-size: 11px; font-weight: 700; letter-spacing: 0.02em;
+  font-variant-numeric: tabular-nums;
+  color: rgb(var(--v-theme-on-accent, 0, 0, 0));
+  background: rgb(var(--v-theme-accent));
+  border-radius: 999px;
+  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.3);
+  pointer-events: none;
+}
 .fc-artistcard__previews img {
   display: block;
   width: 100%; height: 100%;