refactor(audit): extend activity feed row/DTO with count and happenedAtUntil (singletons default)

Prepares the activity feed data shape for session-style rollup (#285). Adds two
new fields that carry null-operation defaults for the existing hour-truncated
dedupe query:

- count: int (required) — always 1 for singleton rows
- happenedAtUntil: OffsetDateTime (nullable) — end-of-session timestamp for
  future rollup rows; null for singletons

No behavioral change yet — the rollup SQL rewrite lands in a follow-up commit.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/test/java/org/raddatz/familienarchiv/dashboard/AuditLogQueryRepositoryIntegrationTest.java

@@ -56,6 +56,8 @@ class AuditLogQueryRepositoryIntegrationTest {
         assertThat(rows.get(0).getKind()).isEqualTo("ANNOTATION_CREATED");
         assertThat(rows.get(0).getDocumentId()).isEqualTo(DOC_ID);
         assertThat(rows.get(0).getHappenedAt()).isNotNull();
+        assertThat(rows.get(0).getCount()).isEqualTo(1);
+        assertThat(rows.get(0).getHappenedAtUntil()).isNull();
     }
 
     @Test

backend/src/test/java/org/raddatz/familienarchiv/dashboard/DashboardServiceTest.java

@@ -104,6 +104,8 @@ class DashboardServiceTest {
             public UUID getDocumentId() { return docId; }
             public Instant getHappenedAt() { return Instant.now(); }
             public boolean isYouMentioned() { return false; }
+            public int getCount() { return 1; }
+            public Instant getHappenedAtUntil() { return null; }
         };
     }
 }