perf(upload): optimize file hot paths and incremental admin stats

- Use RWMutex for disk cache reads and singleflight for WebP cache misses
- Cache migration read-only state and file access whitelist with pub/sub invalidation
- Add w_upload_stats incremental counters updated on upload/delete
- Add w_uploads composite indexes and goose backfill migrations
- Document performance analysis in docs/PERFORMANCE.md
- Fix RegisterCustomRoutes to accept apiV1Router parameter

internal/testhelper/test_helper.go

@@ -42,6 +42,7 @@ func SetupTestEnvironment(t *testing.T) (*gorm.DB, *miniredis.Miniredis, func())
 		&model.ExternalAccount{},
 		&model.SystemConfig{},
 		&model.Upload{},
+		&model.UploadStat{},
 		&model.TaskExecution{},
 		&model.Template{},
 		&model.AccessToken{},