feat(timeline): add EventType enum

PERSONAL/HISTORICAL classify a curated timeline event. The string value
names are a stable frontend styling contract (family vs. muted world
accent) — no mapping layer; renaming requires a coordinated frontend
change. First piece of the new timeline domain (Zeitstrahl, issue #774).

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

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

@@ -0,0 +1,17 @@
+package org.raddatz.familienarchiv.timeline;
+
+/**
+ * Kind of a curated {@link TimelineEvent}.
+ *
+ * <p>The string value names are a <strong>stable frontend styling contract</strong>: the
+ * Svelte timeline components hard-code {@code "PERSONAL"} (family accent) and
+ * {@code "HISTORICAL"} (muted world accent) as Tailwind class-map keys. There is no
+ * mapping layer — renaming either value requires a coordinated frontend change. See
+ * ADR-040.
+ */
+public enum EventType {
+    /** A family/personal event (birth, wedding, move) — rendered with the family accent. */
+    PERSONAL,
+    /** A world/historical event providing context — rendered with the muted world accent. */
+    HISTORICAL
+}