feat(timeline): add TimelineEventRequest input DTO

Flat input DTO with Bean Validation (@NotBlank/@NotNull/@Size). createdBy/
updatedBy deliberately absent (server-populated; CWE-639). version is an
optional concurrency token, exempt from the server-only audit rule. Per #775.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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

@@ -0,0 +1,40 @@
+package org.raddatz.familienarchiv.timeline;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotNull;
+import jakarta.validation.constraints.Size;
+
+import org.raddatz.familienarchiv.document.DatePrecision;
+
+import java.time.LocalDate;
+import java.util.List;
+import java.util.UUID;
+
+/**
+ * Flat input DTO for creating/updating a {@link TimelineEvent}. Bean Validation fires at the
+ * controller boundary (via {@code @Valid}) and produces a 400 {@code VALIDATION_ERROR} for the
+ * presence/size constraints below; cross-field rules (the RANGE invariant), date normalization,
+ * id dedupe, and the title-length structured-error guard live in {@code TimelineEventService}.
+ *
+ * <p><strong>{@code createdBy}/{@code updatedBy} are intentionally absent.</strong> Authorship is
+ * server-populated from the session principal only — accepting it from the body would be an
+ * authorship-forgery / mass-assignment vector (CWE-639; see ADR-040 §7).
+ *
+ * @param version optional optimistic-lock concurrency token (the {@code @Version} the client last
+ *                saw), applied on <em>update</em> only. This is a concurrency token, <strong>not</strong>
+ *                an authorship field, so it is deliberately exempt from the §7 server-only audit rule.
+ *                Null on update means "no concurrency check" (last-write-wins). No range validation —
+ *                a stale/negative value is simply a mismatch the lock rejects at flush; the lock, not
+ *                a validator, is the control.
+ */
+public record TimelineEventRequest(
+        @NotBlank @Size(max = 255) String title,
+        @NotNull EventType type,
+        @NotNull LocalDate eventDate,
+        DatePrecision precision,
+        LocalDate eventDateEnd,
+        @Size(max = 5000) String description,
+        Long version,
+        @Size(max = 50) List<UUID> personIds,
+        @Size(max = 50) List<UUID> documentIds
+) {}