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/router/v1/custom.go

@@ -10,7 +10,7 @@ import (
 )
 
 // RegisterCustomRoutes registers custom business routes to keep routing clean and stable.
-func RegisterCustomRoutes(_ *gin.RouterGroup) {
+func RegisterCustomRoutes(apiV1Router *gin.RouterGroup) {
 	customRouter := apiV1Router.Group("/custom")
 	{
 		customRouter.GET("/hello", custom.Hello)