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>

docs/ConfigurationGuide.md

@@ -304,6 +304,7 @@ DocuElevate can automatically pull document attachments from IMAP mailboxes —
 | `IMAP1_SSL`                   | Use SSL (`true`/`false`).                                   | `true`            |
 | `IMAP1_POLL_INTERVAL_MINUTES` | Frequency in minutes to poll for new mail.                  | `5`               |
 | `IMAP_READONLY_MODE`          | When `true`, fetches and processes attachments but does **not** modify the mailbox (no starring, labeling, deleting, or flag changes). Use for pre-production instances sharing a mailbox with production. Default: `false`. | `false` |
+| `IMAP_ATTACHMENT_FILTER`      | Controls which attachment types are ingested from emails. `documents_only` (default) ingests PDFs and office files only — images are skipped. `all` ingests every supported file type including images. Individual per-user IMAP accounts can override this global default. | `documents_only` |
 
 #### Per-User IMAP Integrations