fix(hotfix): v3.13.1 — fix ImportError in api_list_sites (My Sites broken)

routes.py still imported MAX_SITES_PER_USER from core.site_api which was
removed in v3.13.0. This caused 500 errors on /api/sites, making My Sites
appear empty for all users. Fixed to use get_cached_max_sites().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "mcphub-server"
-version = "3.13.0"
+version = "3.13.1"
 description = "AI-native management hub for WordPress, WooCommerce, and self-hosted services via Model Context Protocol (MCP)"
 authors = [
     {name = "MCP Hub", email = "contact@mcphub.dev"}