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/tests/api_test.rs

@@ -297,6 +297,12 @@ impl PersonCommand for PanicPersonCommand {
     async fn upsert_batch(&self, _: &[Person]) -> Result<(), DomainError> {
         panic!()
     }
+    async fn backfill_from_credits_batch(
+        &self,
+        _batch_size: u32,
+    ) -> Result<(u64, bool), DomainError> {
+        panic!()
+    }
 }
 
 struct PanicPersonQuery;
@@ -317,6 +323,13 @@ impl PersonQuery for PanicPersonQuery {
     async fn list_orphaned_persons(&self) -> Result<Vec<PersonId>, DomainError> {
         panic!()
     }
+    async fn list_page(
+        &self,
+        _limit: u32,
+        _offset: u32,
+    ) -> Result<Vec<domain::models::Person>, DomainError> {
+        panic!()
+    }
 }
 
 struct PanicSearchPort;