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/models/artist.py

@@ -15,7 +15,14 @@ class Artist(Base):
     __tablename__ = "artist"
 
     id: Mapped[int] = mapped_column(Integer, primary_key=True)
-    name: Mapped[str] = mapped_column(String(255), nullable=False, unique=True)
+    # Display name: freely editable, NON-unique (two real creators can share a
+    # name). Decoupled from identity/storage in migration 0077 (#130) — renaming
+    # touches ONLY this. Was unique until then.
+    name: Mapped[str] = mapped_column(String(255), nullable=False)
+    # Storage/identity key: IMMUTABLE + unique. This is the on-disk path
+    # component (download_service artist_slug = artist.slug → images_root/<slug>/
+    # <platform>/…), so it is set once at creation (collision-suffixed) and NEVER
+    # changes — a rename must not move files. Existing artists keep their slug.
     slug: Mapped[str] = mapped_column(String(255), nullable=False, unique=True)
     notes: Mapped[str | None] = mapped_column(Text, nullable=True)