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/utils/settings_service.py

@@ -1425,6 +1425,18 @@ SETTING_METADATA = {
         "required": False,
         "restart_required": False,
     },
+    "imap_attachment_filter": {
+        "category": "IMAP",
+        "description": (
+            "Controls which attachment types are ingested from IMAP emails. "
+            "Accepted values: 'documents_only' (PDFs and office files only, default) or 'all' (including images). "
+            "Per-user IMAP accounts can override this global default."
+        ),
+        "type": "string",
+        "sensitive": False,
+        "required": False,
+        "restart_required": False,
+    },
     # Monitoring - Uptime Kuma
     "uptime_kuma_url": {
         "category": "Monitoring",