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

tests/test_api_artists_create.py

@@ -47,3 +47,28 @@ async def test_autocomplete_empty_query(client):
     resp = await client.get("/api/artists/autocomplete?q=")
     assert resp.status_code == 200
     assert await resp.get_json() == []
+
+
+@pytest.mark.asyncio
+async def test_patch_renames_display_name_only(client):
+    # #130: PATCH renames the display name; slug unchanged (paths safe).
+    created = await (await client.post("/api/artists", json={"name": "12345678"})).get_json()
+    resp = await client.patch(
+        f"/api/artists/{created['id']}", json={"name": "Kurotsuchi Machi"}
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["name"] == "Kurotsuchi Machi"
+    assert body["slug"] == created["slug"]        # slug frozen
+
+
+@pytest.mark.asyncio
+async def test_patch_rename_validation(client):
+    created = await (await client.post("/api/artists", json={"name": "Nn"})).get_json()
+    assert (await client.patch(
+        f"/api/artists/{created['id']}", json={"name": "  "}
+    )).status_code == 400
+    assert (await client.patch(
+        f"/api/artists/{created['id']}", json={})).status_code == 400
+    assert (await client.patch(
+        "/api/artists/999999", json={"name": "Ghost"})).status_code == 404