feat(timeline): add DerivedEventType enum and TimelineEntryDTO record

DerivedEventType: BIRTH / DEATH / MARRIAGE discriminator for derived events.
TimelineEntryDTO: unified String-id DTO for both curated and derived events;
id is String (not UUID) to accommodate synthetic prefixed ids (birth:/death:/marriage:).

Refs #776
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -0,0 +1,8 @@
+package org.raddatz.familienarchiv.timeline;
+
+/** Discriminator for derived life-events assembled from Person / PersonRelationship data. */
+public enum DerivedEventType {
+    BIRTH,
+    DEATH,
+    MARRIAGE
+}

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

@@ -0,0 +1,31 @@
+package org.raddatz.familienarchiv.timeline;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+import org.raddatz.familienarchiv.document.DatePrecision;
+
+import java.time.LocalDate;
+
+/**
+ * Unified DTO for timeline entries — covers both curated {@link TimelineEvent} rows
+ * ({@code derived=false}) and derived life-events assembled from Person/relationship data
+ * ({@code derived=true}).
+ *
+ * <p>The {@code id} field is typed {@code String}, not {@code UUID}, because derived events
+ * carry synthetic prefixed ids ({@code birth:{uuid}}, {@code death:{uuid}},
+ * {@code marriage:{uuid}}) that are structurally non-UUID by construction. Any write endpoint
+ * must reject ids that do not parse as {@code UUID} — enforced and tested in issue #5.
+ *
+ * <p>Callers of {@code TimelineService.assembleDerivedEvents()} must independently enforce
+ * {@code READ_ALL} authorization before invoking that method (see ADR-043).
+ */
+public record TimelineEntryDTO(
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String id,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) EventType type,
+        LocalDate eventDate,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) DatePrecision precision,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) boolean derived,
+        DerivedEventType derivedType,
+        String primaryPersonName,
+        String relatedPersonName
+) {
+}