Add find_missing_ebooks scheduled job

Introduce a safety-net scheduled job that scans completed audiobooks and auto-triggers ebook fetches for missing companions. Changes include:

- New Prisma migration + schema field: requests.ebook_auto_retry_count (nullable) to track lifetime auto-retries.
- New processor: src/lib/processors/find-missing-ebooks.processor.ts implementing the scan (limit 50), gating by ebook_auto_grab_enabled and source flags, creating ebook child requests or retrying failed ones up to a cap of 5, using transactions for race-safety and rolling back the counter if enqueue fails.
- Job queue integration: add job type, payload, processor registration, and addFindMissingEbooksJob helper.
- Scheduler integration: register the scheduled job (daily midnight) and trigger path.
- Documentation updates: backend scheduler and ebook-sidecar docs describing behavior and limits.
- Tests: add comprehensive unit tests for the processor and update scheduler tests and job-queue test helper.

This implements automated recovery for missing ebook companions while keeping the retry counter processor-private and ensuring safe concurrency handling.

tests/services/scheduler.service.test.ts

@@ -16,6 +16,7 @@ const jobQueueMock = vi.hoisted(() => ({
   addAudibleRefreshJob: vi.fn(),
   addRetryMissingTorrentsJob: vi.fn(),
   addRetryFailedImportsJob: vi.fn(),
+  addFindMissingEbooksJob: vi.fn(),
   addCleanupSeededTorrentsJob: vi.fn(),
   addMonitorRssFeedsJob: vi.fn(),
   addSyncShelvesJob: vi.fn(),
@@ -80,7 +81,7 @@ describe('SchedulerService', () => {
     const service = new SchedulerService();
     await service.start();
 
-    expect(prismaMock.scheduledJob.create).toHaveBeenCalledTimes(9);
+    expect(prismaMock.scheduledJob.create).toHaveBeenCalledTimes(10);
     expect(jobQueueMock.addRepeatableJob).toHaveBeenCalledWith(
       'audible_refresh',
       { scheduledJobId: 'job-1' },
@@ -289,6 +290,7 @@ describe('SchedulerService', () => {
     ['audible_refresh', 'addAudibleRefreshJob'],
     ['retry_missing_torrents', 'addRetryMissingTorrentsJob'],
     ['retry_failed_imports', 'addRetryFailedImportsJob'],
+    ['find_missing_ebooks', 'addFindMissingEbooksJob'],
     ['cleanup_seeded_torrents', 'addCleanupSeededTorrentsJob'],
     ['monitor_rss_feeds', 'addMonitorRssFeedsJob'],
     ['sync_reading_shelves', 'addSyncShelvesJob'],