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/src/catalog/queries/search_assets.rs

@@ -14,6 +14,11 @@ pub struct SearchAssetsQuery {
     pub offset: u32,
 }
 
+pub struct SearchResult {
+    pub items: Vec<Asset>,
+    pub total: u64,
+}
+
 pub struct SearchAssetsHandler {
     asset_repo: Arc<dyn AssetRepository>,
 }
@@ -23,9 +28,15 @@ impl SearchAssetsHandler {
         Self { asset_repo }
     }
 
-    pub async fn execute(&self, query: SearchAssetsQuery) -> Result<Vec<Asset>, DomainError> {
-        self.asset_repo
+    pub async fn execute(&self, query: SearchAssetsQuery) -> Result<SearchResult, DomainError> {
+        let total = self
+            .asset_repo
+            .count_search(&query.owner_id, &query.filters)
+            .await?;
+        let items = self
+            .asset_repo
             .search(&query.owner_id, &query.filters, query.limit, query.offset)
-            .await
+            .await?;
+        Ok(SearchResult { items, total })
     }
 }