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/inbox/inbox.service.ts

@@ -232,7 +232,7 @@ export class InboxService {
     variant: 'preview' | 'thumbnail',
     preferredUsername: string | null,
   ): Promise<string> {
-    const { doc } = await this.resolveDocument(id, preferredUsername);
+    const { doc, pdfPath } = await this.resolveDocument(id, preferredUsername);
     if (!Number.isInteger(page) || page < 1 || page > doc.PageCount) {
       throw new NotFoundException('Seite nicht gefunden');
     }
@@ -244,7 +244,12 @@ export class InboxService {
     try {
       await fs.access(filePath);
     } catch {
-      throw new NotFoundException('Seite nicht gefunden');
+      try {
+        await this.barcodeScanner.ensurePageCache(doc.Id, pdfPath);
+        await fs.access(filePath);
+      } catch {
+        throw new NotFoundException('Seite nicht gefunden');
+      }
     }
     return filePath;
   }