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.

backend/app/models/__init__.py

@@ -2,6 +2,7 @@
 
 from .app_setting import AppSetting
 from .artist import Artist
+from .artist_visit import ArtistVisit
 from .backup_run import BackupRun
 from .base import Base
 from .credential import Credential
@@ -28,6 +29,7 @@ __all__ = [
     "Base",
     "AppSetting",
     "Artist",
+    "ArtistVisit",
     "BackupRun",
     "Source",
     "Credential",

backend/app/models/artist_visit.py

@@ -0,0 +1,36 @@
+"""ArtistVisit — per-artist 'last viewed' timestamp.
+
+Powers the "+N new since last visit" badge on the artists directory and
+the matching banner on `ArtistView`. One row per artist, single global
+operator. When the multi-user model lands, the PK widens to
+`(user_id, artist_id)` — currently aspirational only (no User model,
+no services/access.py); operator approved skipping `user_id` for now
+under rule #22 (breaking changes welcome).
+
+Seed at migration time: every existing artist gets `last_viewed_at = NOW()`
+so the badge starts at 0 across the board (no noisy "5000 unseen" on
+first deploy). New artists also auto-get a row via
+`ArtistService.find_or_create`.
+"""
+
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Integer, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from .base import Base
+
+
+class ArtistVisit(Base):
+    __tablename__ = "artist_visit"
+
+    artist_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("artist.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    last_viewed_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        server_default=func.now(),
+    )