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/mappers/users.rs

@@ -4,7 +4,10 @@ use domain::ports::RemoteActorInfo;
 use template_askama::{RemoteActorData, RemoteActorDisplay, UserSummaryView};
 
 pub fn user_summary_view(u: &UserSummary) -> UserSummaryView {
-    let name = u.email().split('@').next().unwrap_or("?").to_string();
+    let name = u
+        .display_name()
+        .map(String::from)
+        .unwrap_or_else(|| u.username().to_string());
     let initial = name.chars().next().unwrap_or('?').to_ascii_uppercase();
     let avg_display = u
         .avg_rating