Proxy kiwix-serve through this app instead of linking to it directly

map.priesol.net sits behind a reverse proxy that only forwards to
maptool itself, so the previous approach (publish kiwix-serve's port,
link the browser straight at it) had no address a real browser could
ever reach — not just a Docker-internal hostname as first assumed, but
no host/port combination at all works from outside.

GET /kiwix/* now proxies to kiwix-serve (Kiwix.proxy in
lib/maptool/kiwix.rb, a plain Net::HTTP passthrough), and the kiwix
service runs with --urlRootLocation=/kiwix so every link/asset it
emits already carries that same prefix and needs no rewriting
(confirmed against libkiwix/internalServer.cpp, which strips this
exact prefix off incoming requests). Drops the port-publishing
approach from the previous commit entirely.

docs/development.adoc

@@ -80,12 +80,12 @@ Wikipedia https://library.kiwix.org[ZIM] loaded — set `KIWIX_URL` to
 wherever *the server* can reach it (`docker-compose.yaml` already runs
 a `kiwix` service pointed at `data/kiwix/wikipedia.zim` and wires
 `KIWIX_URL` to it via its compose service name; blank/unset disables
-the link, same as every other optional integration here). This is
-used only for the server-side reachability check — a browser can't
-resolve a Docker-internal hostname like `kiwix`, so the link itself is
-built from `KIWIX_PUBLIC_PORT` (the host port kiwix-serve is
-`ports:`-published on, `8090` in `docker-compose.yaml`) plus whatever
-host the visitor already used to reach maptool itself. Not part of the
+the link, same as every other optional integration here). Used only
+server-side: production sits behind a reverse proxy that forwards only
+to this app, so there's no address a browser could reach kiwix-serve
+at directly — `GET /kiwix/*` (`app.rb`) proxies to it instead, and the
+`kiwix` service runs with `--urlRootLocation=/kiwix` so its own
+emitted links/assets already carry that same prefix. Not part of the
 repo — a ZIM is a large, self-contained archive, obtain one locally:
 
 * https://library.kiwix.org/#lang=eng&category=wikipedia[Kiwix's own
@@ -105,10 +105,11 @@ repo — a ZIM is a large, self-contained archive, obtain one locally:
 
 Once it's there, `docker compose up -d kiwix` (or a full restart)
 picks it up — `GET /api/kiwix` reports `available: true` and the
-"Nearby on Wikipedia" link appears. kiwix-serve's own geo search
-(`GET /search?pattern=&latitude=&longitude=&distance=`,
-`lib/maptool/kiwix.rb`'s docstring) is linked to directly, not
-proxied/embedded — it already has a full search UI of its own.
+"Nearby on Wikipedia" link appears, pointed at kiwix-serve's own geo
+search (`GET /kiwix/search?pattern=&latitude=&longitude=&distance=`)
+through this app's own proxy rather than at kiwix-serve directly — it
+already has a full search UI of its own, this just makes it reachable
+through the same origin/reverse proxy as the rest of the app.
 
 == GNSS position (gpsd)