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/stores/devices.ts

@@ -26,7 +26,7 @@ export const useDevicesStore = defineStore('devices', () => {
     }
   }
 
-  async function updateDevice(id: number, patch: Partial<Pick<Device, 'name' | 'orientation' | 'rotationIntervalMinutes' | 'wakeTimes' | 'timezone' | 'uniquenessWindow'>>) {
+  async function updateDevice(id: number, patch: Partial<Pick<Device, 'name' | 'orientation' | 'rotationIntervalMinutes' | 'wakeTimes' | 'timezone' | 'uniquenessWindow' | 'rotationMode' | 'prioritizeNeverShown'>>) {
     const res = await fetch(`/api/devices/${id}`, {
       method: 'PATCH',
       headers: { 'Content-Type': 'application/json' },