Update DownlinkQueue operations RateLimit keys

CHANGELOG.md

@@ -13,6 +13,8 @@ For details about compatibility between different releases, see the **Commitment
 
 ### Changed
 
+- Rate limiting for downlink queue operations (`DownlinkQueuePush`, `DownlinkQueueReplace`) is now applied at the application level instead of per-device. This may result in more `ResourceExhausted` (429) errors when multiple devices under the same application perform downlink queue operations concurrently.
+
 ### Deprecated
 
 ### Removed

pkg/ttnpb/application_interfaces.go

@@ -210,3 +210,13 @@ func (m *DownlinkQueueRequest) IDString() string {
 	}
 	return ids.IDString()
 }
+
+// RateLimitKey implements ratelimit.Keyer.
+// Returns an application-level key to enable rate limiting per application for downlink queue operations.
+func (m *DownlinkQueueRequest) RateLimitKey() string {
+	ids := m.EndDeviceIds
+	if ids == nil {
+		return ""
+	}
+	return "application:" + ids.GetApplicationIds().IDString()
+}