feat(translation): re-translate on model change — artist-scoped + global re-run (#146)

retranslate_posts resets the 5 translation columns to NULL for a scoped set of posts (all, or WHERE artist_id IN ids) then reuses the untranslated sweep to re-run them, chasing the tail until drained (run-until-done). Interpreter cache keys on engine_version so a changed model re-translates, an unchanged one is cache-fast. Reset only happens when the service is configured+healthy so translations are never wiped when they can't be rebuilt. New POST /settings/translation/retranslate (artist_id | all=true). UI: per-artist 'Re-translate posts' on the Artist Management tab + 'Re-translate all' in the Settings Translation card, both with confirm dialogs. No migration (reuses m143 columns).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/artist/ArtistManagementTab.vue

@@ -101,6 +101,47 @@
       </v-card>
     </v-dialog>
 
+    <section class="fc-artist-mgmt__sec">
+      <h2 class="fc-h2">Translation</h2>
+      <p class="fc-muted text-body-2 mb-3">
+        Re-translate every post by {{ overview.name }} through the current
+        Interpreter model — clears the stored translations and rebuilds them.
+        Use this after switching translation models to refresh just this artist.
+      </p>
+      <div class="d-flex align-center flex-wrap" style="gap: 10px;">
+        <v-btn
+          size="small" variant="tonal" rounded="pill"
+          prepend-icon="mdi-translate"
+          :loading="retranslating" :disabled="!translationEnabled"
+          @click="confirmRetranslate = true"
+        >Re-translate posts</v-btn>
+        <span v-if="!translationEnabled" class="fc-muted text-caption">
+          Translation is off — enable it in Settings → Maintenance.
+        </span>
+      </div>
+    </section>
+
+    <!-- m146: per-artist re-translate — clears + rebuilds this artist's stored
+         translations (used after a translation-model change). -->
+    <v-dialog v-model="confirmRetranslate" max-width="440">
+      <v-card>
+        <v-card-title class="fc-h2">Re-translate {{ overview.name }}?</v-card-title>
+        <v-card-text class="text-body-2">
+          Clears the stored translation for every post by
+          <strong>{{ overview.name }}</strong> and re-runs them through your
+          current Interpreter model. Unchanged text comes back from the cache
+          instantly. Runs in the background until done.
+        </v-card-text>
+        <v-card-actions>
+          <v-spacer />
+          <v-btn variant="text" @click="confirmRetranslate = false">Cancel</v-btn>
+          <v-btn color="accent" :loading="retranslating" @click="onRetranslate">
+            Re-translate
+          </v-btn>
+        </v-card-actions>
+      </v-card>
+    </v-dialog>
+
     <section class="fc-artist-mgmt__sec">
       <h2 class="fc-h2">Danger zone</h2>
       <ArtistDangerZone
@@ -113,9 +154,11 @@
 </template>
 
 <script setup>
-import { computed, ref } from 'vue'
+import { computed, onMounted, ref } from 'vue'
 import { useRouter, RouterLink } from 'vue-router'
 
+import { useApi } from '../../composables/useApi.js'
+import { useImportStore } from '../../stores/import.js'
 import { useSourcesStore } from '../../stores/sources.js'
 import { toast } from '../../utils/toast.js'
 import ArtistDangerZone from './ArtistDangerZone.vue'
@@ -125,8 +168,38 @@ const props = defineProps({
 })
 
 const router = useRouter()
+const api = useApi()
+const importStore = useImportStore()
 const sources = useSourcesStore()
 
+// m146: per-artist re-translate. Gated on translation being enabled globally
+// (the endpoint 400s otherwise) — read the shared import settings once.
+const translationEnabled = ref(false)
+const retranslating = ref(false)
+const confirmRetranslate = ref(false)
+
+onMounted(async () => {
+  try {
+    await importStore.loadSettings()
+    translationEnabled.value = !!importStore.settings?.translation_enabled
+  } catch { /* non-fatal — the button just stays disabled */ }
+})
+
+async function onRetranslate () {
+  retranslating.value = true
+  try {
+    await api.post('/api/settings/translation/retranslate', {
+      body: { artist_id: props.overview.id },
+    })
+    confirmRetranslate.value = false
+    toast({ text: `Re-translating ${props.overview.name}…`, type: 'success' })
+  } catch (e) {
+    toast({ text: `Re-translate failed: ${e.message}`, type: 'error' })
+  } finally {
+    retranslating.value = false
+  }
+}
+
 // #130: move a source into another artist.
 const moveOpen = ref(false)
 const moveSource = ref(null)