proxy: use wherever http.Client is, update mautrix

Added a new common.ConfigurableTransport interface which mediabrowser,
ombi, jellyseer, discord, telegram and matrix (i.e.
ThirdPartService/ContactMethodLinker) now all implement. proxies are
bound to them in main.go, Email is still a special case (but from the
previous commit, mailgun does use the proxy).

mautrix/go has been updated, and context.TODO()s stuck everywhere since
I still don't really comprehend why I should use them (FIXME literally).

jellyseerr/jellyseerr.go

@@ -55,6 +55,11 @@ func NewJellyseerr(server, key string, timeoutHandler co.TimeoutHandler) *Jellys
 	}
 }
 
+// SetTransport sets the http.Transport to use for requests. Can be used to set a proxy.
+func (js *Jellyseerr) SetTransport(t *http.Transport) {
+	js.httpClient.Transport = t
+}
+
 func (js *Jellyseerr) req(mode string, uri string, data any, queryParams url.Values, headers map[string]string, response bool) (string, int, error) {
 	var params []byte
 	if data != nil {