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/application/tests/catalog/queries/get_timeline.rs

@@ -28,7 +28,7 @@ async fn returns_paginated_assets() {
 
     let handler = GetTimelineHandler::new(asset_repo, meta_repo);
 
-    let page = handler
+    let result = handler
         .execute(GetTimelineQuery {
             owner_id: owner,
             caller_id: None,
@@ -38,7 +38,8 @@ async fn returns_paginated_assets() {
         .await
         .unwrap();
 
-    assert_eq!(page.len(), 3);
+    assert_eq!(result.items.len(), 3);
+    assert_eq!(result.total, 5);
 }
 
 #[tokio::test]
@@ -48,7 +49,7 @@ async fn returns_empty_for_no_assets() {
 
     let handler = GetTimelineHandler::new(asset_repo, meta_repo);
 
-    let page = handler
+    let result = handler
         .execute(GetTimelineQuery {
             owner_id: SystemId::new(),
             caller_id: None,
@@ -58,5 +59,6 @@ async fn returns_empty_for_no_assets() {
         .await
         .unwrap();
 
-    assert!(page.is_empty());
+    assert!(result.items.is_empty());
+    assert_eq!(result.total, 0);
 }