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/file_management.go

@@ -140,6 +140,7 @@ func DeleteFile(c *gin.Context) {
 		c.JSON(http.StatusOK, response.Err(ErrDeleteFileFailed))
 		return
 	}
+	recordUploadStatsRemove(ctx, &upload)
 	c.JSON(http.StatusOK, response.OKNil())
 }
 
@@ -290,6 +291,7 @@ func DeleteMyFile(c *gin.Context) {
 		c.JSON(http.StatusOK, response.Err(ErrDeleteFileFailed))
 		return
 	}
+	recordUploadStatsRemove(ctx, &upload)
 	c.JSON(http.StatusOK, response.OKNil())
 }