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/app.py

@@ -80,6 +80,12 @@ def check_route_tags(
     return None
 
 
+@app.route("/docs")
+def docs() -> ResponseReturnValue:
+    """Render the API documentation page."""
+    return render_template("api.html")
+
+
 @app.route("/")
 def index() -> ResponseReturnValue:
     """Render the landing page with no relation loaded."""