feat: add upload policies, daily limits, and storage quotas

- Add environment variables to configure anonymous uploads, daily upload caps, and default user storage limits.
- Update config loader to parse and validate the new settings.
- Implement backend logic to track daily usage and active storage per user.
- Update README and `.env.example` to document the new settings and admin panels.

backend/libs/handlers/api_docs.go

@@ -20,7 +20,7 @@ type apiDocsData struct {
 }
 
 func (a *App) APIDocs(w http.ResponseWriter, r *http.Request) {
-	a.renderer.Render(w, http.StatusOK, "api.html", web.PageData{
+	a.renderPage(w, r, http.StatusOK, "api.html", web.PageData{
 		Title:       "API documentation",
 		Description: "Curl and ShareX upload examples for Warpbox.",
 		Data: apiDocsData{