feat(artist): editable display name + rename surface; drop name-uniqueness (#130 step 1)

First step of decoupling artist identity/storage/display. migration 0077 drops
uq_artist_name so the display name is free text (two genuinely different creators
can share a name); the slug stays the immutable, unique storage/identity key (the
on-disk path component — untouched, so nothing moves). ArtistService.rename +
PATCH /api/artists/<id> change the name ONLY. Frontend: inline pencil-edit on the
artist header (mirrors TagCard), slug/route unaffected so no navigation. Fixes the
operator's 'no surface to rename an artist' + the name-collision fragility.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/services/artist_service.py

@@ -283,6 +283,23 @@ class ArtistService:
             await self.session.commit()
         return artist, created
 
+    async def rename(self, artist_id: int, name: str) -> Artist | None:
+        """Change the display NAME only (#130). The slug — and every on-disk path
+        keyed off it — is IMMUTABLE, so a rename never moves files or risks a path
+        collision. Name is free text and NON-unique (migration 0077). Returns the
+        updated Artist, None if not found; raises ValueError on empty name."""
+        cleaned = (name or "").strip()
+        if not cleaned:
+            raise ValueError("artist name must not be empty")
+        artist = (await self.session.execute(
+            select(Artist).where(Artist.id == artist_id)
+        )).scalar_one_or_none()
+        if artist is None:
+            return None
+        artist.name = cleaned
+        await self.session.commit()
+        return artist
+
     async def autocomplete(self, prefix: str, limit: int = 20) -> list[Artist]:
         cleaned = (prefix or "").strip()
         if not cleaned: