proxy server: collapse low-priority push notifications

When a device is unreachable (deep doze, offline), FCM queues
non-collapsible messages individually, with a hard limit of 100
pending messages per device. A Jami device that is a member of a few
active swarms receives a sustained stream of connection-churn and
expiration notifications (observed: 27-44/min, ~12k/day), so a phone
suspended for a few tens of minutes is enough to overflow the queue.
Once full, FCM silently drops messages, including the high-priority
notification announcing an incoming call: the callee never wakes up
and the caller gets a timeout after the ICE window (observed in
production, with the call notification never delivered while pushes
sent after the device woke up arrived in under a second).

Mark Android notifications as collapsible (collapse_key) when they
are neither high-priority (calls, messages, invitations - as set by
the client through Value::priority) nor resubscribe requests. While
collapsed messages are pending, FCM only keeps the last one per key,
so routine sync noise can no longer evict actionable notifications.

A single constant key is used instead of one key per DHT listen key:
FCM only honors 4 distinct collapse keys per device, and the Android
client reacts to any low-priority notification the same way (wake up
and resynchronize against the proxy), so finer granularity would not
change client behavior.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/dht_proxy_server.cpp

@@ -1267,6 +1267,21 @@ DhtProxyServer::sendPushNotification(
                 Json::Value androidConfig(Json::objectValue);
                 androidConfig["priority"] = priority;
                 androidConfig["ttl"] = "86400s"; // time to live = 24 hours
+                // Make routine sync notifications (connection churn, value
+                // expirations) collapsible: FCM keeps only the last pending
+                // message per collapse key when the device is unreachable
+                // (dozing/offline), while non-collapsible messages pile up in
+                // a 100-message queue. When that queue overflows, FCM silently
+                // drops messages - including the high-priority ones that
+                // announce incoming calls. A single constant key is used
+                // because FCM only honors 4 distinct collapse keys per device
+                // and the Android client reacts to any of these notifications
+                // the same way: wake up and resynchronize against the proxy.
+                // High-priority notifications (calls, messages, invites - see
+                // Value::priority set by the client) and resubscribe requests
+                // remain non-collapsible so none of them can be overwritten.
+                if (!highPriority && !isResubscribe)
+                    androidConfig["collapse_key"] = "sync";
                 notification["android"] = std::move(androidConfig);
             } else {
                 notification["priority"] = priority;