Add API docs page and update README/AGENTS

- Add /docs route serving web/templates/api.html: full Bootstrap 5
  documentation page covering all three API endpoints with parameter
  tables, example requests, and example responses
- Add 'API docs' link to the navbar on the main map page
- Update README.md: add web frontend section with feature list, dev
  server instructions, and API endpoint summary table
- Update AGENTS.md: add web/ layout, API endpoint table, Flask run
  instructions, and route_master example relation IDs

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

web/templates/index.html

@@ -13,6 +13,7 @@
 
 <nav class="navbar navbar-dark bg-dark px-3" style="height:56px">
   <a class="navbar-brand" href="/">OSM Public Transport → GeoJSON</a>
+  <a class="nav-link text-white" href="/docs">API docs</a>
 </nav>
 
 <div class="container-fluid h-100 p-0">