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

frontend/src/stores/artist.js

@@ -73,6 +73,16 @@ export const useArtistStore = defineStore('artist', () => {
     }
   }
 
+  // Rename the DISPLAY name only (#130). The slug is immutable, so the route
+  // (slug-based) and on-disk paths are unaffected — just patch the shown name.
+  async function rename (name) {
+    if (!overview.value) return
+    const id = overview.value.id
+    const body = await api.patch(`/api/artists/${id}`, { body: { name } })
+    if (overview.value && overview.value.id === id) overview.value.name = body.name
+    return body
+  }
+
   const hasMoreImages = computed(() => !started || nextCursor.value !== null)
   const postCount = computed(() => overview.value?.post_count ?? null)
   const imageCount = computed(() => overview.value?.image_count ?? null)
@@ -81,6 +91,6 @@ export const useArtistStore = defineStore('artist', () => {
   return {
     overview, images, loading, imagesLoading, error, notFound,
     hasMoreImages, postCount, imageCount, lastAdded,
-    load, loadMoreImages,
+    load, loadMoreImages, rename,
   }
 })