feat(heads): nightly auto-retrain + inline Retrain button in Explore

Two cadences for keeping heads in sync with your tagging:
- PASSIVE: a nightly `scheduled_train_heads` beat (skips if a run is already
  in flight; creates+commits the run row before dispatching train_heads so the
  ml worker always finds it). Folds the day's accepts/rejects + newly-eligible
  concepts into the heads without anyone clicking.
- ACTIVE: a "Retrain heads" button in the Explore trail bar — bank the +/-
  feedback you just gave while walking content, without a trip to Settings.

Shared logic in a new useHeadTraining composable (trigger + poll + start/finish
toasts), used by the Explore button; reflects an already-running run (incl. the
nightly one) on mount.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

frontend/src/views/ExploreView.vue

@@ -30,11 +30,22 @@
         >
           <img :src="c.thumbnail_url" alt="" loading="lazy" />
         </button>
-        <v-btn
-          class="fc-ex__reseed" size="small" variant="text" color="accent"
-          prepend-icon="mdi-shuffle-variant" :loading="seeding"
-          @click="reseed"
-        >Random image</v-btn>
+        <div class="fc-ex__trail-actions">
+          <!-- Active retrain right where you tag: fold the +/- you just gave
+               into the heads without a trip to Settings (the nightly beat is the
+               passive cadence). -->
+          <v-btn
+            size="small" variant="text" color="accent"
+            prepend-icon="mdi-brain" :loading="headsBusy || headsRunning"
+            title="Retrain the concept heads on your latest tags"
+            @click="trainHeads"
+          >{{ headsRunning ? 'Training…' : 'Retrain heads' }}</v-btn>
+          <v-btn
+            size="small" variant="text" color="accent"
+            prepend-icon="mdi-shuffle-variant" :loading="seeding"
+            @click="reseed"
+          >Random image</v-btn>
+        </div>
       </nav>
 
       <v-alert v-if="store.error" type="error" variant="tonal" class="ma-3">
@@ -115,6 +126,7 @@ import { useRoute, useRouter } from 'vue-router'
 import { useApi } from '../composables/useApi.js'
 import { useExploreStore } from '../stores/explore.js'
 import { useModalStore } from '../stores/modal.js'
+import { useHeadTraining } from '../composables/useHeadTraining.js'
 import { isTextEntry } from '../utils/textEntry.js'
 import ImageCanvas from '../components/modal/ImageCanvas.vue'
 import ImageMetaBar from '../components/modal/ImageMetaBar.vue'
@@ -132,6 +144,13 @@ const seeding = ref(false)
 const seedError = ref(null)
 const tagPanelRef = ref(null)
 
+// Inline head-retrain (shared with the Settings card) so banking your latest
+// +/- feedback is one click while you walk content.
+const {
+  running: headsRunning, busy: headsBusy, train: trainHeads,
+  start: startHeads, stop: stopHeads,
+} = useHeadTraining()
+
 // Auto-focus the tag input after any action so tagging needs no extra click —
 // the whole point of the workspace (operator-asked 2026-06-26). nextTick waits
 // for the post-navigation re-render, then rAF lands the focus AFTER paint so a
@@ -216,8 +235,14 @@ function onKeyDown (ev) {
   const id = ev.key === 'ArrowLeft' ? store.backTarget() : store.forwardTarget()
   if (id != null) { ev.preventDefault(); goTo(id) }
 }
-onMounted(() => document.addEventListener('keydown', onKeyDown))
-onUnmounted(() => document.removeEventListener('keydown', onKeyDown))
+onMounted(() => {
+  document.addEventListener('keydown', onKeyDown)
+  startHeads()   // reflect a run already in flight (e.g. the nightly one)
+})
+onUnmounted(() => {
+  document.removeEventListener('keydown', onKeyDown)
+  stopHeads()
+})
 </script>
 
 <style scoped>
@@ -250,7 +275,9 @@ onUnmounted(() => document.removeEventListener('keydown', onKeyDown))
 .fc-ex__crumb img { width: 100%; height: 100%; object-fit: cover; }
 .fc-ex__crumb--current { border-color: rgb(var(--v-theme-accent)); }
 .fc-ex__crumb:focus-visible { outline: 2px solid rgb(var(--v-theme-accent)); outline-offset: 1px; }
-.fc-ex__reseed { margin-left: auto; }
+.fc-ex__trail-actions {
+  margin-left: auto; display: flex; align-items: center; gap: 4px; flex: 0 0 auto;
+}
 
 /* The three panes fill the remaining height; each scrolls on its own.
    grid-template-rows: minmax(0, 1fr) BOUNDS the single row to the container