feat(rotation): per-device image-selection preferences

Adds two settings exposed in the PWA frame-settings sheet:

- rotationMode (enum: random | least_recently_shown | oldest_upload |
  newest_upload). Default oldest_upload preserves the legacy
  hard-coded sort, so existing devices behave identically until the
  user changes it.
- prioritizeNeverShown (bool). When set, the candidate set is narrowed
  to never-shown images first (if any exist) before the mode runs —
  useful for "burn through new uploads before re-shuffling the catalog."

RotationService pipeline:
  1. Pull approved/ready pool.
  2. Drop the last `uniquenessWindow` served (existing).
  3. If prioritizeNeverShown AND any candidates have never been served,
     narrow to those.
  4. Apply the selection mode.

Backend: enum, entity columns + accessors, migration, serializer,
PATCH validator. Frontend: types, stores, settings sheet section
(dropdown + checkbox), test fixtures, save-flow test.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/views/HomeView.vue

@@ -192,6 +192,29 @@
       </p>
     </div>
 
+    <div class="home-view__sheet-field">
+      <p class="home-view__sheet-label">Image selection</p>
+
+      <select
+        class="home-view__mode-select"
+        v-model="editRotationMode"
+        aria-label="Image selection mode"
+      >
+        <option value="oldest_upload">Oldest upload first</option>
+        <option value="newest_upload">Newest upload first</option>
+        <option value="least_recently_shown">Least recently shown</option>
+        <option value="random">Random</option>
+      </select>
+
+      <label class="home-view__rotation-checkbox">
+        <input
+          type="checkbox"
+          v-model="editPrioritizeNeverShown"
+        />
+        <span>Show never-shown images first</span>
+      </label>
+    </div>
+
     <BaseButton
       variant="primary"
       class="home-view__sheet-save"
@@ -465,6 +488,8 @@ const editFrequencyMode = ref<FrequencyMode>('interval')
 const editWakeTimes = ref<number[]>([])
 const editIntervalMinutes = ref<number>(60)
 const editTimezone = ref('UTC')
+const editRotationMode = ref<Device['rotationMode']>('oldest_upload')
+const editPrioritizeNeverShown = ref<boolean>(false)
 
 // Default candidates tried (in order) when adding a new time slot — picks the
 // first one that isn't already in the list, so repeated +Add gives a sensible
@@ -602,6 +627,8 @@ function onEdit(deviceId: number) {
   editIntervalMinutes.value = device.rotationIntervalMinutes
   editWakeTimes.value = [...device.wakeTimes]
   editFrequencyMode.value = device.wakeTimes.length > 0 ? 'times' : 'interval'
+  editRotationMode.value = device.rotationMode
+  editPrioritizeNeverShown.value = device.prioritizeNeverShown
   sheetOpen.value = true
 }
 
@@ -613,6 +640,8 @@ async function saveSettings() {
       name: editName.value.trim() || editingDevice.value.name,
       orientation: editOrientation.value,
       timezone: editTimezone.value,
+      rotationMode: editRotationMode.value,
+      prioritizeNeverShown: editPrioritizeNeverShown.value,
     }
     if (editFrequencyMode.value === 'times') {
       patch.wakeTimes = [...editWakeTimes.value]
@@ -908,6 +937,24 @@ async function saveSettings() {
     color: var(--color-text);
   }
 
+  &__rotation-checkbox {
+    margin-top: var(--space-3);
+    display: flex;
+    align-items: center;
+    gap: var(--space-2);
+    font-size: var(--text-sm);
+    cursor: pointer;
+    min-height: var(--touch-min);
+
+    input[type="checkbox"] {
+      // Bigger than browser default so it remains a comfortable touch target.
+      width: 22px;
+      height: 22px;
+      cursor: pointer;
+      accent-color: var(--color-primary);
+    }
+  }
+
   &__propagation-note {
     margin-top: var(--space-1);
     font-size: var(--text-xs);