feat: search reindex, worker improvements, person IDs, user display names

- add admin POST /api/v1/admin/reindex-search endpoint + event-driven handler
- backfill persons from movie_cast/movie_crew into persons table
- paginate person list_page/backfill_from_credits_batch to cap memory
- concurrent worker event dispatch with semaphore (max 8)
- graceful worker shutdown (drain in-flight tasks on SIGINT)
- always ack events, log handler errors as warnings (no infinite retry)
- NATS ack_wait 600s, AtomicBool guard against concurrent reindex
- add username/display_name to UserSummaryDto and users list
- add person_id to CastMemberDto/CrewMemberDto via get_movie_profile use case
- add movie_id to wrapup MovieRef, person_id to wrapup PersonStat
- thread tmdb_person_id through wrapup cast pipeline
- add is_federated to FeedEntryDto
- cap orphaned persons query with LIMIT 500
- add SPA link to classic site footer

crates/presentation/src/routes.rs

@@ -432,6 +432,10 @@ fn api_routes(rate_limit: u64) -> Router<AppState> {
         .route(
             "/wrapups/{id}/video",
             routing::get(handlers::wrapup::get_video),
+        )
+        .route(
+            "/admin/reindex-search",
+            routing::post(handlers::api::post_reindex_search),
         );
 
     #[cfg(feature = "federation")]