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>

app/models.py

@@ -439,6 +439,11 @@ class UserImapAccount(Base):
     # When True, emails are deleted from the mailbox after their attachments are processed
     delete_after_process = Column(Boolean, nullable=False, default=False)
 
+    # Override for which attachment types to ingest.
+    # NULL means "inherit the global imap_attachment_filter setting".
+    # Allowed values: 'documents_only', 'all'
+    attachment_filter = Column(String(50), nullable=True, default=None)
+
     # When False the account is not polled by the periodic task (but not deleted)
     is_active = Column(Boolean, nullable=False, default=True)