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>

AGENTS.md

@@ -4,9 +4,8 @@ Guidelines for AI coding agents working in this repository.
 
 ## Project overview
 
-A collection of Python CLI tools for working with OpenStreetMap data. Each
-tool is a standalone Python script using Click for the CLI interface and
-Requests for HTTP calls.
+A collection of Python tools for working with OpenStreetMap public transport
+data. Includes a CLI tool and a Flask web frontend with a JSON API.
 
 ## Repository layout
 
@@ -15,10 +14,20 @@ pyproject.toml              - Package metadata and build configuration
 src/
   osm_geojson/
     __init__.py             - Top-level package marker
+    py.typed                - Marker for mypy typed package
     pt/
       __init__.py           - Public transport subpackage
       core.py               - Data fetching and processing functions
       cli.py                - Click CLI commands
+web/
+  app.py                    - Flask application (routes + API endpoints)
+  templates/
+    index.html              - Main map UI
+    api.html                - API documentation page (served at /docs)
+  static/
+    app.js                  - Frontend JavaScript
+    style.css               - CSS for the map UI
+    favicon.svg             - SVG favicon
 tests/
   fixtures/                 - Saved OSM API responses for offline testing
   test_osm_pt_geojson.py    - Test suite
@@ -35,10 +44,13 @@ point registered in `pyproject.toml` under `[project.scripts]`.
 - Python 3.11+
 - CLI via [Click](https://click.palletsprojects.com/)
 - HTTP via [Requests](https://requests.readthedocs.io/)
+- Web frontend via [Flask](https://flask.palletsprojects.com/) 3.x
 - Parse XML with lxml if needed; prefer the OSM JSON API where possible
-- Errors go to stderr; data output goes to stdout
+- Errors go to stderr; data output goes to stdout (CLI)
 - GeoJSON output uses `ensure_ascii=False`
 - All modules, functions, and test functions must have docstrings
+- Library code (`core.py`) raises `OsmError` instead of calling `sys.exit()`;
+  the CLI and Flask views catch `OsmError` and handle it appropriately
 
 ## OSM API
 
@@ -51,6 +63,30 @@ https://www.openstreetmap.org/api/0.6/
 No authentication is required for read-only access. Include a descriptive
 `User-Agent` header in all requests.
 
+## Web frontend API endpoints
+
+| Endpoint | Description |
+|---|---|
+| `GET /api/route/<id>` | Full route GeoJSON, stop list, and sibling routes |
+| `GET /api/segment/<id>?from=NAME&to=NAME[&stops=0]` | Segment between two named stops |
+| `GET /api/route_master/<id>` | All member routes of a route_master |
+| `GET /docs` | API documentation page |
+
+All API responses are JSON. Errors return `{"error": "<code>", "message": "<text>"}`.
+
+## Running the web frontend
+
+```
+cd web
+flask --app app.py run
+```
+
+The web dependencies (Flask) are not in `pyproject.toml`; install separately:
+
+```
+pip install flask
+```
+
 ## Type checking
 
 All code uses type hints. Run mypy in strict mode to check:
@@ -74,7 +110,9 @@ API. Fixture data is stored in `tests/fixtures/` as saved API responses.
 
 | ID       | Description                             |
 |----------|------------------------------------------|
-| 15083963 | M11 Istanbul Metro (subway)              |
+| 15083963 | M11 Istanbul Metro (subway, direction 1) |
+| 15083964 | M11 Istanbul Metro (subway, direction 2) |
+| 15083966 | M11 Istanbul Metro (route_master)        |
 | 18892969 | Bus A1: Bristol Airport → Bus Station    |
 
 ## Dependencies