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/config.py

@@ -571,6 +571,17 @@ class Settings(BaseSettings):
         ),
     )
 
+    imap_attachment_filter: str = Field(
+        default="documents_only",
+        description=(
+            "Controls which attachment types are ingested from IMAP emails. "
+            "Accepted values: "
+            "'documents_only' – ingest only PDFs and office files (Word, Excel, PowerPoint, ODT, etc.); "
+            "'all' – ingest all supported file types including images. "
+            "This is the global default; individual user IMAP accounts can override it."
+        ),
+    )
+
     # Batch processing settings
     processall_throttle_threshold: int = Field(
         default=20,