Add HTMX script and first partial endpoints

Setup HTMX infrastructure:

1. Add HTMX script tag to pageHead (CDN initially, can vendor later) 2. Create /partials/* routes in API type 3. Implement GET /partials/recent-activity (returns HTML fragment) 4. Implement GET /partials/ready-count (returns just the count/badge) 5. Add hx-get + hx-trigger='every 10s' to Recent Activity div 6. Add hx-get + hx-trigger='every 5s' to Ready Queue count in navbar/dashboard