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/admin/system_config/routers.go

@@ -11,6 +11,7 @@ import ("context"
 	"net/http"
 	"time"
 
+	"github.com/Rain-kl/Wavelet/internal/apps/upload"
 	"github.com/Rain-kl/Wavelet/internal/db"
 	"github.com/Rain-kl/Wavelet/internal/model"
 	"github.com/Rain-kl/Wavelet/internal/storage"
@@ -255,9 +256,15 @@ func UpdateSystemConfig(c *gin.Context) {
 	}
 
 	if key == model.ConfigKeyStorageConfig {
+		upload.ResetAccessCaches()
+		upload.PublishAccessCacheInvalidation(c.Request.Context())
 		storage.ResetCache()
 		storage.PublishCacheInvalidation(c.Request.Context())
 	}
+	if key == model.ConfigKeyFileAccessWhitelist {
+		upload.ResetAccessCaches()
+		upload.PublishAccessCacheInvalidation(c.Request.Context())
+	}
 
 	c.JSON(http.StatusOK, response.OKNil())
 }