fix(timeline): mark always-populated audit fields REQUIRED in OpenAPI schema

createdBy/updatedBy are NOT NULL and createdAt/updatedAt/version are Hibernate-
populated on every persisted row, so per the CLAUDE.md rule they must carry
@Schema(requiredMode = REQUIRED) like id/title/type/eventDate/precision already
do. Keeps the generated TypeScript types honest if the entity ever reaches the
OpenAPI spec (responses in #775 are planned as views, per ADR-040).

Extends the #774 task list (which named only the five domain fields) per PR #816 review.

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

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

@@ -99,9 +99,11 @@ public class TimelineEvent {
      * CWE-639 — see ADR-040).
      */
     @Column(name = "created_by", nullable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private UUID createdBy;
 
     @CreationTimestamp
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private LocalDateTime createdAt;
 
     /**
@@ -112,9 +114,11 @@ public class TimelineEvent {
      * stale. Same forgery rationale as {@link #createdBy}.
      */
     @Column(name = "updated_by", nullable = false)
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private UUID updatedBy;
 
     @UpdateTimestamp
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private LocalDateTime updatedAt;
 
     /**
@@ -125,5 +129,6 @@ public class TimelineEvent {
      * internals (CWE-209).
      */
     @Version
+    @Schema(requiredMode = Schema.RequiredMode.REQUIRED)
     private Long version;
 }