feat: frontend-ready backend — pagination, auto-derivatives, list endpoints, bulk ops, OpenAPI

Pagination: count_by_owner + count_search on AssetRepository,
timeline/search return real total count (not page len).

Auto-derivatives: worker enqueues GenerateDerivative when
ExtractMetadata job completes, closing the upload→thumbnail gap.

List endpoints: GET /albums, GET /stacks with user scoping.
ListAlbumsHandler, ListStacksHandler, find_by_owner on AssetStackRepository.

Tag filtering: tag_name field on AssetFilters, JOIN asset_tags+tags
in postgres search/count queries.

Bulk operations: POST /assets/bulk-delete, POST /assets/bulk-tag.

Album update: PUT /albums/{id} with UpdateAlbumHandler (title, description).

OpenAPI: utoipa annotations on all 47 endpoints + all request/response
schemas registered. Scalar UI at /scalar covers full API.

crates/domain/src/catalog/entities.rs

@@ -63,6 +63,7 @@ pub struct AssetFilters {
     pub date_from: Option<DateTimeStamp>,
     pub date_to: Option<DateTimeStamp>,
     pub is_processed: Option<bool>,
+    pub tag_name: Option<String>,
 }
 
 // --- AssetMetadata ---