fix(metadata): handle unhashable list values in metadata split (#13116)

### What problem does this PR solve?

This PR fixes missing metadata on documents synced from the Moodle
connector, especially for **Book** modules.

Background:
- Moodle Book metadata includes fields like `chapters`, which is a
`list[dict]`.
- During metadata normalization in
`DocMetadataService._split_combined_values`, list deduplication used
`dict.fromkeys(...)`.
- `dict.fromkeys(...)` fails for unhashable values (like `dict`),
causing metadata update to fail.
- Result: documents were imported, but metadata was not saved for
affected module types (notably Books).

What this PR changes:
- Replaces hash-based list deduplication with `dedupe_list(...)`, which
safely handles unhashable list items while preserving order.
- This allows Book metadata (and other complex list metadata) to be
persisted correctly.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [ ] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

Contribution during my time at RAGcon GmbH.

api/db/services/doc_metadata_service.py

@@ -231,8 +231,9 @@ class DocMetadataService:
                             new_values.append(item)
                     else:
                         new_values.append(item)
-                # Remove duplicates while preserving order
-                processed[key] = list(dict.fromkeys(new_values))
+                # Remove duplicates while preserving order.
+                # Use string-based dedupe to support unhashable values (e.g. dict entries).
+                processed[key] = dedupe_list(new_values)
             else:
                 processed[key] = value