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/apps/upload/cleanup.go

@@ -110,6 +110,7 @@ func (h *SystemCleanupHandler) Execute(ctx context.Context, _ []byte) (*task.Tas
 				continue
 			}
 
+			recordUploadStatsRemove(ctx, &u)
 			totalDeleted++
 			lastID = u.ID
 		}