feat(audit): add findRecentContributorsForDocuments query (max 4, recency order)

Adds a window-function query that returns at most 4 contributors per document
ordered by most-recent activity. Used by DocumentService to populate the
contributors field in DocumentSearchItem (issue #281).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -106,4 +106,38 @@ public interface AuditLogQueryRepository extends JpaRepository<AuditLog, UUID> {
             ORDER BY a.document_id, MIN(a.happened_at)
             """, nativeQuery = true)
     List<ContributorRow> findContributorsPerDocument(@Param("documentIds") List<UUID> documentIds);
+
+    @Query(value = """
+            SELECT
+                ranked.document_id   AS documentId,
+                ranked.actorInitials AS actorInitials,
+                ranked.actorColor    AS actorColor,
+                ranked.actorName     AS actorName
+            FROM (
+                SELECT
+                    a.document_id,
+                    CASE
+                        WHEN u.first_name IS NOT NULL AND u.last_name IS NOT NULL
+                            THEN UPPER(LEFT(u.first_name, 1)) || UPPER(LEFT(u.last_name, 1))
+                        WHEN u.first_name IS NOT NULL THEN UPPER(LEFT(u.first_name, 1))
+                        WHEN u.last_name  IS NOT NULL THEN UPPER(LEFT(u.last_name, 1))
+                        ELSE '?'
+                    END                                                        AS actorInitials,
+                    COALESCE(u.color, '')                                      AS actorColor,
+                    NULLIF(CONCAT_WS(' ', u.first_name, u.last_name), '')     AS actorName,
+                    ROW_NUMBER() OVER (
+                        PARTITION BY a.document_id
+                        ORDER BY MAX(a.happened_at) DESC
+                    )                                                          AS rn
+                FROM audit_log a
+                LEFT JOIN users u ON u.id = a.actor_id
+                WHERE a.kind IN ('ANNOTATION_CREATED', 'TEXT_SAVED', 'BLOCK_REVIEWED')
+                  AND a.document_id IN :documentIds
+                  AND a.actor_id IS NOT NULL
+                GROUP BY a.document_id, a.actor_id, u.first_name, u.last_name, u.color
+            ) ranked
+            WHERE ranked.rn <= 4
+            ORDER BY ranked.document_id, ranked.rn
+            """, nativeQuery = true)
+    List<ContributorRow> findRecentContributorsForDocuments(@Param("documentIds") List<UUID> documentIds);
 }