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/components/artist/ArtistHeader.vue

@@ -1,8 +1,21 @@
 <template>
   <header class="fc-artist-header">
     <div class="fc-artist-header__left">
-      <h1 class="fc-artist-header__name">{{ name }}</h1>
-      <span v-if="stats" class="fc-artist-header__stats">{{ stats }}</span>
+      <template v-if="!editing">
+        <h1 class="fc-artist-header__name">{{ name }}</h1>
+        <v-icon
+          class="fc-artist-header__edit" size="16" icon="mdi-pencil"
+          title="Rename artist (display name only)" @click="startEdit"
+        />
+      </template>
+      <v-text-field
+        v-else
+        v-model="draft"
+        density="compact" variant="outlined" hide-details autofocus
+        class="fc-artist-header__edit-field"
+        @keydown.enter="submit" @keydown.esc="cancel" @blur="cancel"
+      />
+      <span v-if="stats && !editing" class="fc-artist-header__stats">{{ stats }}</span>
     </div>
     <v-tabs
       :model-value="modelValue"
@@ -34,7 +47,7 @@
 </template>
 
 <script setup>
-import { computed } from 'vue'
+import { computed, ref } from 'vue'
 import { formatLocalDate } from '../../utils/date.js'
 
 const props = defineProps({
@@ -45,7 +58,23 @@ const props = defineProps({
   modelValue:  { type: String, required: true },
 })
 
-defineEmits(['update:modelValue'])
+const emit = defineEmits(['update:modelValue', 'rename'])
+
+// Inline rename (display name only — #130). Mirrors TagCard's pencil-edit.
+const editing = ref(false)
+const draft = ref('')
+function startEdit () {
+  draft.value = props.name
+  editing.value = true
+}
+function cancel () {
+  editing.value = false
+}
+function submit () {
+  const next = draft.value.trim()
+  editing.value = false
+  if (next && next !== props.name) emit('rename', next)
+}
 
 const stats = computed(() => {
   const parts = []
@@ -111,6 +140,20 @@ const stats = computed(() => {
   text-overflow: ellipsis;
 }
 
+.fc-artist-header__edit {
+  flex: 0 0 auto;
+  opacity: 0;
+  cursor: pointer;
+  color: rgb(var(--v-theme-on-surface-variant));
+  transition: opacity .15s ease;
+}
+.fc-artist-header__left:hover .fc-artist-header__edit { opacity: .7; }
+.fc-artist-header__edit:hover { opacity: 1; }
+.fc-artist-header__edit-field {
+  max-width: 340px;
+  font-family: 'Fraunces', Georgia, serif;
+}
+
 .fc-artist-header__tabs {
   flex: 0 0 auto;
 }