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 asks /ip/ about the visitor's own address. 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

@@ -57,8 +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. A card shows the `curl` command that looks
-  up the visitor's own address at `/ip/{addr}`, and that command's live answer.
+  address, and a search of AS names. A card shows the `curl` command that asks
+  `/ip/` about the visitor's own address, with 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

TODO.md

@@ -34,8 +34,8 @@ tested.
 # Completed Steps
 
 - 2026-10-27: the home page has an IP information API card. It shows the `curl`
-  command that looks up the visitor's own address at `/ip/{addr}`, and that
-  command's live answer (closes #156)
+  command that asks `/ip/` about the visitor's own address, and the live answer
+  to that request (closes #156)
 - 2026-10-27: on the AS page, a click on the heading of the IPv4 or the IPv6
   prefix list folds or unfolds the list. Both lists start open (closes #155)
 - 2026-10-27: the AS page lists its IPv4 and its IPv6 prefixes by mask length,