feat: auto-move imported emails to IMAP folder and add 90-day cleanup

- New ImapFolderService moves emails to configurable "importiert" folder
  after successful import, creating the folder if it doesn't exist
- Daily cron at 03:00 moves emails older than 90 days to trash and empties it
- Extract createImapClient() helper in EmailDownloadService
- Add ensurePageCache() with in-flight deduplication to BarcodeScannerService
- InboxService regenerates page cache on-demand when image file is missing
- IMAP_IMPORTED_FOLDER and IMAP_TRASH_FOLDER added to .env.example and docker-compose

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

paperless-backend/src/email/email.module.ts

@@ -9,6 +9,7 @@ import { EmailPageCacheService } from './email-page-cache.service';
 
 import { EmailImportController } from './email-import.controller';
 import { EmailImportService } from './email-import.service';
+import { ImapFolderService } from './imap-folder.service';
 import { CorrespondentEmailMapping } from '../database/entities/correspondent-email-mapping.entity';
 import { Task } from '../database/entities/task.entity';
 import { PreprocessingModule } from '../preprocessing/preprocessing.module';
@@ -26,7 +27,7 @@ import { PreprocessingModule } from '../preprocessing/preprocessing.module';
     PreprocessingModule,
   ],
   controllers: [EmailController, EmailImportController],
-  providers: [EmailImportService, EmailPageCacheService],
+  providers: [EmailImportService, EmailPageCacheService, ImapFolderService],
   exports: [EmailPageCacheService],
 })
 export class EmailModule {}