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>

docker-compose.yml

@@ -36,6 +36,8 @@ services:
       - IMAP_USE_SSL=${IMAP_USE_SSL:-true}
       - IMAP_USERNAME=${IMAP_USERNAME:-}
       - IMAP_PASSWORD=${IMAP_PASSWORD:-}
+      - IMAP_IMPORTED_FOLDER=${IMAP_IMPORTED_FOLDER:-importiert}
+      - IMAP_TRASH_FOLDER=${IMAP_TRASH_FOLDER:-Trash}
       - BELEGNUMMER_GET_URL=${BELEGNUMMER_GET_URL:-}
       - BELEGNUMMER_SET_URL=${BELEGNUMMER_SET_URL:-}
       - AGRARMONITOR_BASE_URL=${AGRARMONITOR_BASE_URL:-https://admin7.agrarmonitor.de}