Show the IP information API on the home page (closes #156)

The home page gets an IP information API card. It shows the curl
command that looks up the visitor's own address at /ip/{addr}. The page
then makes that same request and shows the JSON answer, pretty-printed.
An error answer, such as a 404 for an address with no route, is shown
the same way. A request that fails shows an error line.

The server gives the page the visitor's address, found as /ip/ finds
it, so a trusted proxy's X-Forwarded-For is honoured. The command takes
its origin from the browser, so it names the host the visitor used.

Model: opus-5-5

README.md

@@ -57,7 +57,8 @@ Every route answers `HEAD` as it answers `GET`, without the body.
 ### Web Interface
 
 - `GET /` - Home page (HTML): headline figures, lookups by AS number and by IP
-  address, and a search of AS names
+  address, and a search of AS names. A card shows the `curl` command that looks
+  up the visitor's own address at `/ip/{addr}`, and that command's live answer.
 - `GET /status` - HTML status dashboard, which reads `/api/v1/stats`
 - `GET /status.json` - JSON statistics: feed, memory, database and route figures
 - `GET /.well-known/healthcheck.json` - Health check (JSON): 200 when the