docs(timeline): pin relative issue ordinals to Gitea issue numbers

The issue body's milestone-relative ordinals ("issue 3", "issue 5") become
unreadable once the milestone closes. Resolved against the Zeitstrahl milestone:
issue 3 = #775 (CRUD API: service/controller/DTO), issue 5 = #777 (assembly
endpoint with the per-person filter). Mapping anchored by issue 6 = #778
(date-label helper) and issue 9 = #781 (curator forms) in #774's forward notes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/timeline/TimelineEvent.java

@@ -122,7 +122,7 @@ public class TimelineEvent {
     private LocalDateTime updatedAt;
 
     /**
-     * Optimistic-lock version for the multi-curator edit flow (issue 3). Object {@code Long}
+     * Optimistic-lock version for the multi-curator edit flow (#775). Object {@code Long}
      * (not primitive) so it is {@code null} before first persist; Hibernate sets {@code 0} on
      * insert. A concurrent-write conflict must be translated to {@code DomainException.conflict}
      * in the service layer (ADR-040) — otherwise it surfaces as HTTP 500 with Hibernate

backend/src/main/java/org/raddatz/familienarchiv/timeline/TimelineEventRepository.java

@@ -5,5 +5,5 @@ import org.springframework.data.jpa.repository.JpaRepository;
 import java.util.UUID;
 
 public interface TimelineEventRepository extends JpaRepository<TimelineEvent, UUID> {
-    // TODO(issue 5): findByPersonsContaining(Person) needed for the per-person filter
+    // TODO(#777): findByPersonsContaining(Person) needed for the per-person filter
 }