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

alembic/versions/0077_artist_name_not_unique.py

@@ -0,0 +1,32 @@
+"""drop uq_artist_name — decouple display name from identity/storage
+
+Revision ID: 0077
+Revises: 0076
+Create Date: 2026-07-04
+
+Artist model fragility fix (milestone #130). One `slug` column was doing
+identity + storage-path + display, and BOTH `name` and `slug` were UNIQUE, so
+the display name couldn't be edited freely and two genuinely different creators
+collided. Decouple: `slug` stays the immutable, unique storage/identity key (the
+on-disk path component — untouched here); `name` becomes freely editable, NON-
+unique display text. This migration only drops the `uq_artist_name` constraint;
+no data moves and no path changes.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+
+revision: str = "0077"
+down_revision: Union[str, None] = "0076"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.drop_constraint("uq_artist_name", "artist", type_="unique")
+
+
+def downgrade() -> None:
+    # Re-adding the UNIQUE would fail if duplicate names now exist; callers that
+    # need to reverse this must dedupe names first.
+    op.create_unique_constraint("uq_artist_name", "artist", ["name"])