feat(update-service): add PATCH /app/{id}/changelog with audit log

Allows editors to update release notes at any time. Every change is
recorded in update_operation_log with action CHANGELOG_UPDATE and
before/after values in detailJson, satisfying the audit requirement.

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

update-service/src/main/java/com/xuqm/update/controller/AppVersionController.java

@@ -384,6 +384,31 @@ public class AppVersionController {
         }
     }
 
+    @PatchMapping("/app/{id}/changelog")
+    public ResponseEntity<ApiResponse<AppVersionEntity>> updateChangeLog(
+            @PathVariable String id,
+            @RequestBody Map<String, Object> body) {
+        AppVersionEntity entity = versionRepository.findById(id).orElseThrow();
+        String oldChangeLog = entity.getChangeLog();
+        Object raw = body.get("changeLog");
+        String newChangeLog = raw != null && !raw.toString().isBlank() ? raw.toString().trim() : null;
+        entity.setChangeLog(newChangeLog);
+        AppVersionEntity saved = versionRepository.save(entity);
+        operationLogService.record(
+                saved.getAppKey(),
+                "APP_VERSION",
+                saved.getId(),
+                "CHANGELOG_UPDATE",
+                null,
+                Map.of(
+                        "versionName", saved.getVersionName(),
+                        "versionCode", saved.getVersionCode(),
+                        "before", oldChangeLog != null ? oldChangeLog : "",
+                        "after", newChangeLog != null ? newChangeLog : ""
+                ));
+        return ResponseEntity.ok(ApiResponse.success(saved));
+    }
+
     @GetMapping("/app/list")
     public ResponseEntity<ApiResponse<List<AppVersionEntity>>> list(
             @RequestParam String appKey, @RequestParam AppVersionEntity.Platform platform) {