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/test/java/org/raddatz/familienarchiv/timeline/TimelineEventServiceTest.java

@@ -153,6 +153,32 @@ class TimelineEventServiceTest {
         verify(events, never()).saveAndFlush(any());
     }
 
+    @Test
+    void create_rejects_RANGE_with_eventDateEnd_before_eventDate() {
+        TimelineEventRequest request = new TimelineEventRequest(
+                "Krieg", EventType.HISTORICAL, LocalDate.of(1918, 11, 11),
+                DatePrecision.RANGE, LocalDate.of(1914, 7, 28), null, null, null, null); // end < start
+
+        assertThatThrownBy(() -> service.create(request, actor))
+                .isInstanceOf(DomainException.class)
+                .extracting(e -> ((DomainException) e).getCode()).isEqualTo(ErrorCode.INVALID_DATE_RANGE);
+        verify(events, never()).saveAndFlush(any());
+    }
+
+    @Test
+    void create_accepts_RANGE_with_eventDateEnd_equal_to_eventDate() {
+        stubFlushSetsVersion();
+        LocalDate sameDay = LocalDate.of(1914, 7, 28);
+        TimelineEventRequest request = new TimelineEventRequest(
+                "Eintägiges Ereignis", EventType.HISTORICAL, sameDay,
+                DatePrecision.RANGE, sameDay, null, null, null, null); // end == start is a valid closed range
+
+        TimelineEventView view = service.create(request, actor);
+
+        assertThat(view.eventDate()).isEqualTo(sameDay);
+        assertThat(view.eventDateEnd()).isEqualTo(sameDay);
+    }
+
     // --- title-length service guard ---
 
     @Test
@@ -287,6 +313,21 @@ class TimelineEventServiceTest {
         assertThat(existing.getDocuments()).isEmpty();
     }
 
+    @Test
+    void update_rejects_RANGE_with_eventDateEnd_before_eventDate_without_saving() {
+        UUID id = UUID.randomUUID();
+        TimelineEvent existing = existingEvent(id, UUID.randomUUID());
+        when(events.findById(id)).thenReturn(Optional.of(existing));
+        TimelineEventRequest request = new TimelineEventRequest(
+                "Krieg", EventType.HISTORICAL, LocalDate.of(1918, 11, 11),
+                DatePrecision.RANGE, LocalDate.of(1914, 7, 28), null, null, null, null); // end < start
+
+        assertThatThrownBy(() -> service.update(id, request, secondEditor))
+                .isInstanceOf(DomainException.class)
+                .extracting(e -> ((DomainException) e).getCode()).isEqualTo(ErrorCode.INVALID_DATE_RANGE);
+        verify(events, never()).saveAndFlush(any());
+    }
+
     @Test
     void update_of_missing_id_throws_TIMELINE_EVENT_NOT_FOUND() {
         UUID id = UUID.randomUUID();