feat(imap): add attachment type filter for IMAP ingestion

Add a configurable switch to control which attachment types are ingested
via IMAP. Images are excluded by default; office files and PDFs are ingested.

- Add global `IMAP_ATTACHMENT_FILTER` config setting (default: `documents_only`)
- Add `attachment_filter` column to `UserImapAccount` model for per-user override
- Migration 032 adds the column to `user_imap_accounts` table
- Update `fetch_attachments_and_enqueue()` to respect filter (documents_only/all)
- Update `pull_inbox()`, `_pull_user_imap_accounts()`, and
  `_pull_user_integration_imap()` to pass the resolved filter
- Update IMAP accounts API (schemas, create/update handlers, response serializer)
- Update IMAP accounts UI to show attachment filter dropdown in modal and
  display filter badges on account cards
- Add 6 new tests covering attachment filter behaviour
- Update ConfigurationGuide.md, EmailIngestion.md, and .env.demo

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

.env.demo

@@ -311,6 +311,12 @@ IMAP2_DELETE_AFTER_PROCESS=false
 # Use for pre-production instances that share a mailbox with production.
 IMAP_READONLY_MODE=false
 
+# Controls which attachment types are ingested from IMAP emails.
+# 'documents_only' (default) – PDFs and office files only; images are skipped.
+# 'all' – all supported file types including images.
+# Per-user IMAP accounts can override this global default.
+IMAP_ATTACHMENT_FILTER=documents_only
+
 # **Storage/Document Services**
 # Amazon S3
 AWS_REGION=us-east-1