feat: add admin backfill-versions endpoint for documents without history

Adds POST /api/admin/backfill-versions which creates an initial snapshot
(editorName="Datenimport", changedFields=[]) for every document that has
no version entry yet, using the document's createdAt as the version timestamp.

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

backend/src/main/java/org/raddatz/familienarchiv/controller/AdminController.java

@@ -1,7 +1,10 @@
 package org.raddatz.familienarchiv.controller;
 
+import org.raddatz.familienarchiv.dto.BackfillResult;
 import org.raddatz.familienarchiv.security.Permission;
 import org.raddatz.familienarchiv.security.RequirePermission;
+import org.raddatz.familienarchiv.service.DocumentService;
+import org.raddatz.familienarchiv.service.DocumentVersionService;
 import org.raddatz.familienarchiv.service.MassImportService;
 import org.springframework.http.ResponseEntity;
 import org.springframework.web.bind.annotation.GetMapping;
@@ -18,6 +21,8 @@ import lombok.RequiredArgsConstructor;
 public class AdminController {
 
     private final MassImportService massImportService;
+    private final DocumentService documentService;
+    private final DocumentVersionService documentVersionService;
 
     @PostMapping("/trigger-import")
     public ResponseEntity<MassImportService.ImportStatus> triggerMassImport() {
@@ -29,4 +34,11 @@ public class AdminController {
     public ResponseEntity<MassImportService.ImportStatus> importStatus() {
         return ResponseEntity.ok(massImportService.getStatus());
     }
+
+    @PostMapping("/backfill-versions")
+    public ResponseEntity<BackfillResult> backfillVersions() {
+        int count = documentVersionService.backfillMissingVersions(
+                documentService.getDocumentsWithoutVersions());
+        return ResponseEntity.ok(new BackfillResult(count));
+    }
 }

backend/src/main/java/org/raddatz/familienarchiv/dto/BackfillResult.java

@@ -0,0 +1,7 @@
+package org.raddatz.familienarchiv.dto;
+
+import io.swagger.v3.oas.annotations.media.Schema;
+
+public record BackfillResult(
+        @Schema(requiredMode = Schema.RequiredMode.REQUIRED) int count
+) {}

backend/src/main/java/org/raddatz/familienarchiv/repository/DocumentRepository.java

@@ -34,6 +34,9 @@ public interface DocumentRepository extends JpaRepository<Document, UUID>, JpaSp
 
     List<Document> findByTags_Id(UUID tagId);
 
+    @Query("SELECT d FROM Document d WHERE d.id NOT IN (SELECT DISTINCT dv.documentId FROM DocumentVersion dv)")
+    List<Document> findDocumentsWithoutVersions();
+
     @Query("SELECT DISTINCT d FROM Document d " +
             "JOIN d.receivers r " +
             "WHERE " +

backend/src/main/java/org/raddatz/familienarchiv/service/DocumentService.java

@@ -257,6 +257,10 @@ public class DocumentService {
                 .orElseThrow(() -> DomainException.notFound(ErrorCode.DOCUMENT_NOT_FOUND, "Document not found: " + id));
     }
 
+    public List<Document> getDocumentsWithoutVersions() {
+        return documentRepository.findDocumentsWithoutVersions();
+    }
+
     public List<Document> getDocumentsBySender(UUID senderId) {
         return documentRepository.findBySenderId(senderId);
     }

backend/src/main/java/org/raddatz/familienarchiv/service/DocumentVersionService.java

@@ -56,6 +56,26 @@ public class DocumentVersionService {
                 .build());
     }
 
+    @Transactional
+    public int backfillMissingVersions(List<Document> docs) {
+        int count = 0;
+        for (Document doc : docs) {
+            List<DocumentVersion> existing = versionRepository.findByDocumentIdOrderBySavedAtAsc(doc.getId());
+            if (!existing.isEmpty()) continue;
+            LocalDateTime savedAt = doc.getCreatedAt() != null ? doc.getCreatedAt() : LocalDateTime.now();
+            versionRepository.save(DocumentVersion.builder()
+                    .documentId(doc.getId())
+                    .savedAt(savedAt)
+                    .editorId(null)
+                    .editorName("Datenimport")
+                    .snapshot(serializeSnapshot(doc))
+                    .changedFields("[]")
+                    .build());
+            count++;
+        }
+        return count;
+    }
+
     public List<DocumentVersionSummary> getSummaries(UUID documentId) {
         return versionRepository.findByDocumentIdOrderBySavedAtAsc(documentId).stream()
                 .map(v -> new DocumentVersionSummary(