feat: add PDF magic bytes validation, 410 Gone, and manual extraction tests (refs #144)

Add filename .pdf extension fallback and %PDF magic bytes validation to
extractManual controller. Update getJobStatus to return 410 Gone for
expired jobs. Add 16 unit tests covering all acceptance criteria.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/src/features/ocr/domain/ocr.service.ts

@@ -368,8 +368,8 @@ export class OcrService {
       return result;
     } catch (error) {
       if (error instanceof JobNotFoundError) {
-        const err: any = new Error(`Job ${jobId} not found. Jobs expire after 1 hour.`);
-        err.statusCode = 404;
+        const err: any = new Error('Job expired (max 2 hours). Please resubmit.');
+        err.statusCode = 410;
         throw err;
       }