rproxy: give its own answers at least 10 seconds to be sent (closes #295)

The write deadline of an answer rproxy gives itself was
SWRP_MAX_RESPONSE_TIME_MS from when the answer started. With a limit
under a second, a host too busy to run rproxy for that long passes the
deadline before the first byte is written. Go then closes the
connection and the visitor gets no answer.

Model: opus-5-5

SPEC.md

@@ -2245,7 +2245,8 @@ These limits apply per request, in both directions:
 - `SWRP_MAX_RESPONSE_TIME_MS` runs from the end of the request to the last byte
   of the answer reaching the visitor. The request ends when the app has been
   sent all of it, or when the app's answer arrives if that is sooner. An answer
-  `rproxy` gives itself has the whole time from when it starts.
+  `rproxy` gives itself has the whole time from when it starts, and never less
+  than 10 seconds.
 
 The largest request line and headers is 32 KiB. A kept-open connection is closed
 after 120 seconds idle. Neither has a setting. traefik, by default, closes an