feat(remote): keep slow remote session requests alive with a heartbeat

Titles, tracks and licence stream a newline every 30s until the body is ready, so a slow service outlasts the client timeout and proxy idle limits; add a per-server timeout.

docs/reference/configuration/services.md

@@ -83,7 +83,7 @@ auth tokens in localStorage rather than in HTTP cookies need. Extraction is read
 Definitions of remote unshackle service servers, used by the `--remote` mode. Each entry is
 named by you (pick it with `--server`, or do not write that flag when you configure only one) and gives
 the server's `url` (required), an optional `api_key`, an optional `auth_headers` list, an
-optional `server_cdm` boolean, and an optional `services` sub-dict of per-service local
+optional `server_cdm` boolean, an optional `timeout` in seconds, and an optional `services` sub-dict of per-service local
 overrides such as `title_map`.
 
 Leave `server_cdm` unset and the client follows the server: it uses the server CDM for each
@@ -97,6 +97,12 @@ keeps the one that works for the rest of the HTTP session. Names you give keep y
 are not repeated in the fallbacks, so unshackle tries
 `auth_headers: ["Authorization", "x-secret-key"]` as `Authorization`, `x-secret-key`, `X-Api-Key`.
 
+`timeout` is the time in seconds that the client waits for data from the server. The
+default is `120`. The server sends a heartbeat every 30 seconds while it gets titles, tracks
+or licences, so a slow service does not cause a timeout. Increase `timeout` only when the
+server runs an older version that sends no heartbeat. Give a number of seconds. With a
+value below 30 the client can time out before the first heartbeat. `0` means the default.
+
 In `--remote` mode unshackle turns the server's service list into synthetic CLI
 commands that operate against it, falling back to the tags in that `services` sub-dict when
 unshackle cannot fetch the list. Each synthetic command carries the server-side service's options and