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.

src/lib/services/job-queue.service.ts

@@ -24,6 +24,7 @@ export type JobType =
   | 'audible_refresh'
   | 'retry_missing_torrents'
   | 'retry_failed_imports'
+  | 'find_missing_ebooks'
   | 'cleanup_seeded_torrents'
   | 'monitor_rss_feeds'
   | 'sync_reading_shelves'
@@ -105,6 +106,10 @@ export interface RetryFailedImportsPayload extends JobPayload {
   scheduledJobId?: string;
 }
 
+export interface FindMissingEbooksPayload extends JobPayload {
+  scheduledJobId?: string;
+}
+
 export interface CleanupSeededTorrentsPayload extends JobPayload {
   scheduledJobId?: string;
 }
@@ -386,6 +391,12 @@ export class JobQueueService {
       return await processRetryFailedImports(payloadWithJobId);
     });
 
+    this.queue.process('find_missing_ebooks', 1, async (job: BullJob<FindMissingEbooksPayload>) => {
+      const { processFindMissingEbooks } = await import('../processors/find-missing-ebooks.processor');
+      const payloadWithJobId = await this.ensureJobRecord(job, 'find_missing_ebooks');
+      return await processFindMissingEbooks(payloadWithJobId);
+    });
+
     this.queue.process('cleanup_seeded_torrents', 1, async (job: BullJob<CleanupSeededTorrentsPayload>) => {
       const { processCleanupSeededTorrents } = await import('../processors/cleanup-seeded-torrents.processor');
       const payloadWithJobId = await this.ensureJobRecord(job, 'cleanup_seeded_torrents');
@@ -756,6 +767,21 @@ export class JobQueueService {
     );
   }
 
+  /**
+   * Add find missing ebooks job
+   */
+  async addFindMissingEbooksJob(scheduledJobId?: string): Promise<string> {
+    return await this.addJob(
+      'find_missing_ebooks',
+      {
+        scheduledJobId,
+      } as FindMissingEbooksPayload,
+      {
+        priority: 7,
+      }
+    );
+  }
+
   /**
    * Add cleanup seeded torrents job
    */