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>

CHANGELOG.md

@@ -19,6 +19,28 @@ accepts both, but the store flags the old spelling as deprecated
 
 ## Unreleased
 
+### Security
+
+- The web interface refuses state-changing requests (`POST`, `PUT`, `PATCH`,
+  `DELETE`) sent by another website's page. Any site a LAN user visited could
+  make their browser submit a plain HTML form to `http://<pi>:5000` -- CORS
+  does not stop such a request, only hides its answer -- and
+  `/api/v3/system/action` accepted form bodies, so that page could reboot or
+  power off the Pi, pull code, or reach any other mutating route. A request
+  whose `Origin` (or, without one, `Referer`) is not the host it was sent to,
+  or is `null`, now gets 403 `CROSS_SITE_REQUEST`
+  (`web_interface/origin_guard.py`). `/api/v3/system/action` also refuses a
+  form-encoded or `text/plain` body (415) unless it carries HTMX's
+  `HX-Request` header; every caller in the interface already sends JSON.
+- **Behaviour change for API scripts:** clients that send no `Origin` or
+  `Referer` -- curl, Python `requests`, Home Assistant, the MQTT bridge --
+  are unaffected. A browser page served from a *different* origin (a
+  dashboard or userscript on another host) can no longer call the mutating
+  API; call it server-side instead. Anyone posting a form body to
+  `system/action` must switch to JSON. Behind a reverse proxy, forward the
+  original `Host` (`proxy_set_header Host $host;`); `X-Forwarded-Host` is
+  not trusted.
+
 ## 3.7.0
 
 Sports consolidation stage 3 (#672). No behaviour change: nothing in core