fix(cctv): serve only live feeds on /media

/api/cctv/media relayed still-image cameras with an ordinary fetch: it followed
redirects to any host, passed through any content type and had no body cap.
Stills are served by /api/cctv/frame, which keeps redirects on the registered
origin, accepts only image/* and caps the body, and no client requests a still
through /media. /media now answers 404 for anything but HLS, MP4/WebM and
MJPEG, before any upstream request.

CHANGELOG.md

@@ -1,5 +1,15 @@
 # Changelog
 
+- Refuse still-image cameras on the CCTV media relay. `/api/cctv/media` is the
+  live-feed route, but it also relayed any registered still camera through an
+  ordinary fetch that followed redirects to any host, passed through whatever
+  content type came back and had no cap on the body, bypassing every protection
+  `/api/cctv/frame` applies to the same URL. Only HLS, MP4, WebM and MJPEG
+  feeds are relayed now; a still camera, or an unknown id, gets a JSON 404
+  without an upstream request and is served only through `/api/cctv/frame`.
+  The client already loaded stills through `/frame`, so nothing it requests
+  changes.
+
 - Stop one retired bikeshare feed from reporting the whole layer as failed.
   City feeds are fetched independently, but failure was recorded in a single
   flag that only a successful *activation* cleared — and an already-loaded city