fix(timeline): reject reversed RANGE events; thread precision

The DB CHECK chk_timeline_event_range enforces only the presence
biconditional (eventDateEnd non-null IFF RANGE), not date ordering, so a
RANGE event with eventDateEnd before eventDate persisted silently and
rendered as a negative span. validateRangeInvariant now also rejects
end-before-start (INVALID_DATE_RANGE); equal dates remain a valid one-day
closed range.

Also compute effectivePrecision once per create/update and thread it into
validateRangeInvariant and applyUpdate instead of recomputing.

Addresses review of #822 (#775).

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

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

@@ -43,9 +43,9 @@ public class TimelineEventService {
 
     @Transactional
     public TimelineEventView create(TimelineEventRequest request, UUID actorId) {
-        validateRangeInvariant(request);
-        validateTitleLength(request);
         DatePrecision precision = effectivePrecision(request);
+        validateRangeInvariant(request, precision);
+        validateTitleLength(request);
 
         TimelineEvent event = TimelineEvent.builder()
                 .title(request.title())
@@ -69,9 +69,10 @@ public class TimelineEventService {
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.TIMELINE_EVENT_NOT_FOUND,
                         "Timeline event not found: " + id));
         requireVersionMatch(request, event);
-        validateRangeInvariant(request);
+        DatePrecision precision = effectivePrecision(request);
+        validateRangeInvariant(request, precision);
         validateTitleLength(request);
-        applyUpdate(event, request, actorId);
+        applyUpdate(event, request, precision, actorId);
 
         // saveAndFlush (not save) so the versioned UPDATE …WHERE version=? fires HERE, inside the
         // try — a bare save() flushes at commit, after this method returns, so the exception would
@@ -108,8 +109,7 @@ public class TimelineEventService {
 
     // --- update mechanics: mutate the managed entity, never reassign collections ---
 
-    private void applyUpdate(TimelineEvent event, TimelineEventRequest request, UUID actorId) {
-        DatePrecision precision = effectivePrecision(request);
+    private void applyUpdate(TimelineEvent event, TimelineEventRequest request, DatePrecision precision, UUID actorId) {
         event.setTitle(request.title());
         event.setType(request.type());
         event.setEventDate(normalizeEventDate(request.eventDate(), precision));
@@ -154,9 +154,15 @@ public class TimelineEventService {
 
     // --- validation / normalization ---
 
-    /** Mirrors the DB biconditional CHECK chk_timeline_event_range — both directions. */
-    private void validateRangeInvariant(TimelineEventRequest request) {
-        boolean isRange = effectivePrecision(request) == DatePrecision.RANGE;
+    /**
+     * Mirrors the DB biconditional CHECK chk_timeline_event_range — both presence directions — and
+     * additionally enforces date ordering, which the DB CHECK does NOT: {@code eventDateEnd} may
+     * equal but never precede {@code eventDate}. Without this guard a reversed range (end before
+     * start) persists silently and renders as a negative span. Equal dates are a valid one-day
+     * closed range.
+     */
+    private void validateRangeInvariant(TimelineEventRequest request, DatePrecision precision) {
+        boolean isRange = precision == DatePrecision.RANGE;
         if (request.eventDateEnd() != null && !isRange) {
             throw DomainException.badRequest(ErrorCode.INVALID_DATE_RANGE,
                     "eventDateEnd is only valid when precision is RANGE");
@@ -165,6 +171,10 @@ public class TimelineEventService {
             throw DomainException.badRequest(ErrorCode.INVALID_DATE_RANGE,
                     "A RANGE event requires a non-null eventDateEnd");
         }
+        if (isRange && request.eventDateEnd().isBefore(request.eventDate())) {
+            throw DomainException.badRequest(ErrorCode.INVALID_DATE_RANGE,
+                    "eventDateEnd must not precede eventDate");
+        }
     }
 
     /**