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/worker/src/main.rs

@@ -8,7 +8,7 @@ use tracing::{error, info, warn};
 use application::processing::{EnqueueJobCommand, ProcessNextJobCommand};
 use domain::entities::JobType;
 use domain::events::DomainEvent;
-use domain::ports::EventConsumer;
+use domain::ports::{EventConsumer, JobRepository};
 use domain::value_objects::StructuredData;
 
 mod config;
@@ -70,6 +70,7 @@ async fn main() -> anyhow::Result<()> {
         registry,
         event_pub.clone(),
     ));
+    let job_repo: Arc<dyn JobRepository> = repos.job.clone();
     let enqueue = Arc::new(build_enqueue_handler(&repos, event_pub));
 
     // ── Shutdown signal ───────────────────────────────────────────────
@@ -180,6 +181,27 @@ async fn main() -> anyhow::Result<()> {
                             error!(error = %e, "event loop: failed to enqueue SyncSidecar");
                         }
                     }
+                    DomainEvent::JobCompleted { job_id, .. } => {
+                        info!(job_id = %job_id, "event loop: JobCompleted → check derivative generation");
+                        (envelope.ack)();
+                        // Look up the job to see if it was ExtractMetadata
+                        if let Ok(Some(job)) = job_repo.find_by_id(job_id).await
+                            && job.job_type == JobType::ExtractMetadata
+                            && let Some(asset_id) = job.target_asset_id
+                        {
+                            info!(asset_id = %asset_id, "event loop: ExtractMetadata done → enqueue GenerateDerivative");
+                            let cmd = EnqueueJobCommand {
+                                job_type: JobType::GenerateDerivative,
+                                priority: 5,
+                                payload: StructuredData::new(),
+                                target_asset_id: Some(asset_id),
+                                batch_id: None,
+                            };
+                            if let Err(e) = enqueue.execute(cmd).await {
+                                error!(error = %e, "event loop: failed to enqueue GenerateDerivative");
+                            }
+                        }
+                    }
                     DomainEvent::JobEnqueued {
                         job_id, job_type, ..
                     } => {