fix(web): refuse cross-site state-changing requests (Origin/Referer check)

The web interface had no CSRF protection, on the reasoning that anyone who
can forge a request on the LAN can also send it directly. That misses the
browser as a confused deputy: any website a LAN user opens can make their
browser POST a plain HTML form to http://<pi>:5000. CORS does not stop that
request, only hides its answer, and /api/v3/system/action accepted form
bodies, so a hostile page could reboot or power off the Pi, pull code, or
reach any other mutating route.

- web_interface/origin_guard.py: an app-wide before_request hook refuses
  POST/PUT/PATCH/DELETE whose Origin (or, without one, Referer) is not the
  host the request was addressed to, and Origin "null", with 403
  CROSS_SITE_REQUEST. Requests with neither header (curl, Home Assistant,
  the MQTT bridge) are not from a browser and pass. Host and port are
  compared, not the scheme, so a TLS proxy that passes Host through works;
  X-Forwarded-Host is not trusted (no ProxyFix).
- /api/v3/system/action refuses a non-JSON body (415) unless HX-Request is
  set; every caller in the interface already sends JSON.
- app.py comment states the real threat model; SECURITY.md,
  REST_API_REFERENCE.md, WEB_INTERFACE_GUIDE.md and CHANGELOG updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/WEB_INTERFACE_GUIDE.md

@@ -412,6 +412,15 @@ The API blueprint (`web_interface/blueprints/api_v3/`) is registered at
 - No authentication is currently implemented
 - Recommended for trusted networks only
 
+**Other websites:**
+- A web page you open elsewhere could otherwise make your browser send
+  commands to the Pi (reboot, update, config changes). The interface refuses
+  any change request whose `Origin`/`Referer` header names a different site
+  (403 `CROSS_SITE_REQUEST`), so use the interface from its own address.
+- Scripts, curl, Home Assistant and the MQTT bridge send no such header and
+  keep working. Behind a reverse proxy, forward the original `Host` header
+  (nginx: `proxy_set_header Host $host;`).
+
 **Best Practices:**
 1. Run on a private network (not exposed to internet)
 2. Use a firewall to restrict access if needed