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/main/java/org/raddatz/familienarchiv/audit/ActivityFeedRow.java

@@ -12,4 +12,6 @@ public interface ActivityFeedRow {
     UUID getDocumentId();
     Instant getHappenedAt();
     boolean isYouMentioned();
+    int getCount();
+    Instant getHappenedAtUntil();
 }

backend/src/main/java/org/raddatz/familienarchiv/audit/AuditLogQueryRepository.java

@@ -39,7 +39,9 @@ public interface AuditLogQueryRepository extends JpaRepository<AuditLog, UUID> {
                     a.document_id                                     AS documentId,
                     a.happened_at                                     AS happened_at,
                     (a.kind = 'MENTION_CREATED'
-                        AND a.payload->>'mentionedUserId' = :currentUserId) AS youMentioned
+                        AND a.payload->>'mentionedUserId' = :currentUserId) AS youMentioned,
+                    1                                                 AS count,
+                    CAST(NULL AS TIMESTAMPTZ)                         AS happenedAtUntil
                 FROM audit_log a
                 LEFT JOIN users u ON u.id = a.actor_id
                 WHERE a.kind IN ('TEXT_SAVED','FILE_UPLOADED','ANNOTATION_CREATED','COMMENT_ADDED','MENTION_CREATED')

backend/src/main/java/org/raddatz/familienarchiv/dashboard/ActivityFeedItemDTO.java

@@ -14,5 +14,7 @@ public record ActivityFeedItemDTO(
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) UUID documentId,
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) String documentTitle,
         @Schema(requiredMode = Schema.RequiredMode.REQUIRED) OffsetDateTime happenedAt,
-        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) boolean youMentioned
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) boolean youMentioned,
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int count,
+        @Nullable OffsetDateTime happenedAtUntil
 ) {}

backend/src/main/java/org/raddatz/familienarchiv/dashboard/DashboardService.java

@@ -130,13 +130,18 @@ public class DashboardService {
                     ? new ActivityActorDTO(row.getActorInitials(), row.getActorColor(), row.getActorName())
                     : null;
             String docTitle = titleCache.getOrDefault(row.getDocumentId(), "");
+            OffsetDateTime happenedAtUntil = row.getHappenedAtUntil() != null
+                    ? row.getHappenedAtUntil().atOffset(ZoneOffset.UTC)
+                    : null;
             return new ActivityFeedItemDTO(
                     org.raddatz.familienarchiv.audit.AuditKind.valueOf(row.getKind()),
                     actor,
                     row.getDocumentId(),
                     docTitle,
                     row.getHappenedAt().atOffset(ZoneOffset.UTC),
-                    row.isYouMentioned()
+                    row.isYouMentioned(),
+                    row.getCount(),
+                    happenedAtUntil
             );
         }).toList();
     }