fix: bound CCTV media bodies with an idle deadline

The media relay only deadlined the wait for upstream response headers. Once
headers arrived the timer stopped, so an upstream that answered and then sent
nothing held both the client response and the upstream socket open for as long
as the camera host allowed. The declared Content-Length ceiling was the only
body bound, and a chunked or length-less body had none.

A 30-second idle deadline now measures the gap between upstream chunks and
tears the stream down through the existing release path. Because a paused pipe
also stops upstream bytes arriving, the deadline reschedules itself whenever
the response is still waiting to drain, so a viewer on a slow link is not read
as a dead camera. Live feeds that keep delivering frames are unaffected.

docs/CURRENT-STATE.md

@@ -587,10 +587,12 @@ their `configured: false` response.
 
 CCTV media waits at most 15 seconds for upstream response headers and returns
 504 on timeout. Its timer stops when headers arrive, so live bodies can continue
-streaming; body idle deadlines are separate from this header deadline. Error
-responses are cancelled. Buffered snapshots have a 16 MiB streaming cap; an
-oversized image remains an upstream miss and uses the normal fallback chain.
-The existing declared media size ceiling remains 64 MiB.
+streaming; a separate 30-second idle deadline then bounds the gap between
+upstream chunks and releases a body that has gone silent. A response that is
+still waiting to drain reschedules that deadline, so a slow client does not read
+as a stalled upstream. Error responses are cancelled. Buffered snapshots have a
+16 MiB streaming cap; an oversized image remains an upstream miss and uses the
+normal fallback chain. The existing declared media size ceiling remains 64 MiB.
 
 
 ## GBFS upstream bounds