fix(assets): fix 12 bugs found during code review

- Fix _hash_file_obj not restoring file position when orig_pos == 0
- Fix validate_path_within_base swallowing its own ValueError
- Guard against tags=None crash in upload_from_temp_path
- Filter is_missing references from list_references_page results
- Include Decimal in metadata filter isinstance check
- Return 403 instead of 404 for PermissionError in update/add_tags routes
- Re-disable seeder after non-blocking start with --disable-assets-autoscan
- Remove duplicate HashMismatchError/DependencyMissingError from schemas_in
- Move _emit_event outside lock in seeder resume() to prevent deadlock
- Fix total_q ignoring owner_id in list_tags_with_usage
- Capture ORM attributes before commit in resolve_asset_for_download
- Remove dead code (if mime_type is None: pass)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/assets/scanner.py

@@ -329,8 +329,6 @@ def build_asset_specs(
                 logging.warning("Failed to hash %s: %s", abs_p, e)
 
         mime_type = metadata.content_type if metadata else None
-        if mime_type is None:
-            pass
         specs.append(
             {
                 "abs_path": abs_p,