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/api/artists.py

@@ -31,6 +31,24 @@ async def create_or_get():
     }), 201
 
 
+@artists_bp.route("/<int:artist_id>", methods=["PATCH"])
+async def rename(artist_id: int):
+    """Rename an artist's DISPLAY NAME (#130). Name only — the slug and every
+    on-disk path stay put, so this is instant and safe. Name is non-unique."""
+    body = await request.get_json()
+    if not isinstance(body, dict) or not isinstance(body.get("name"), str):
+        return jsonify({"error": "invalid_body"}), 400
+    async with get_session() as session:
+        svc = ArtistService(session)
+        try:
+            artist = await svc.rename(artist_id, body["name"])
+        except ValueError as exc:
+            return jsonify({"error": "empty_name", "detail": str(exc)}), 400
+        if artist is None:
+            return jsonify({"error": "not_found"}), 404
+        return jsonify({"id": artist.id, "name": artist.name, "slug": artist.slug})
+
+
 @artists_bp.route("/autocomplete", methods=["GET"])
 async def autocomplete():
     q = request.args.get("q") or ""