M5: push (FCM + ntfy) + offline catch-up + background notifications

Server (gateway-plugin):
- push.py: FCM (HTTP v1 service-account / legacy key) + ntfy backends; NtfyBackend.server_url for app discovery
- adapter.py: push on offline broadcast + high-priority push when live; fcm.register; server_caps.push_ntfy_server; outbox now ALWAYS appends so a reconnecting app catches up on live-delivered frames (fixes empty chat after notification tap / activity recreation)
- protocol.py / ws_server.py / outbox.py / plugin.yaml: M5 frames + env vars

App (Kotlin CMP):
- Protocol.kt: notification / fcm.register / sync frames + push caps
- GatewayClient.kt: hello carries push creds; auto-sync on reconnect
- IrisController.kt: banners, deep-link, notifyMessageIfBackgrounded (system notification on a regular reply when backgrounded)
- Android: PlatformPush, AppBridge, IrisNotifications, NtfyListenerService, IrisFirebaseMessagingService, AndroidPush, AndroidSecureStore
- Desktop: DesktopPush, DesktopSecureStore
- build files + manifest (permissions, services, deep-links)

Tests: 35-test tests/gateway/test_android.py suite passes (incl. new regression test_live_delivered_frame_still_parked_for_sync). E2E verified on device: push fire, reconnect sync, background notification, and message replay after ChatStore reset.

gateway-plugin/protocol.py

@@ -76,6 +76,10 @@ TYPE_MEDIA_OFFER = "media.offer"
 TYPE_MEDIA_PULL = "media.pull"
 TYPE_MEDIA_PULL_END = "media.pull.end"
 
+# Push / notifications (M5)
+TYPE_NOTIFICATION = "notification"
+TYPE_FCM_REGISTER = "fcm.register"
+
 # ---------------------------------------------------------------------------
 # Error codes (``error`` frame payload.code)
 # ---------------------------------------------------------------------------
@@ -96,6 +100,22 @@ ROLE_ASSISTANT = "assistant"
 ROLE_SYSTEM = "system"
 ROLE_CRON = "cron"
 
+# ---------------------------------------------------------------------------
+# Notification kinds (``notification`` frame payload.kind, M5)
+# ---------------------------------------------------------------------------
+
+NOTIF_CHANNEL_CREATED = "channel_created"
+NOTIF_CHANNEL_RENAMED = "channel_renamed"
+NOTIF_CHANNEL_DELETED = "channel_deleted"
+NOTIF_CRON = "cron"
+NOTIF_APPROVAL = "approval"
+NOTIF_CLARIFY = "clarify"
+NOTIF_GENERIC = "generic"
+
+# Kinds that push even when a device is live (the app may be backgrounded;
+# it decides whether to also show an in-app banner).
+HIGH_PRIORITY_NOTIF_KINDS = frozenset({NOTIF_APPROVAL, NOTIF_CLARIFY, NOTIF_CRON})
+
 
 # ---------------------------------------------------------------------------
 # Envelope
@@ -452,6 +472,41 @@ def sync_done(cursor: int, *, id: Optional[int] = None) -> Frame:
     return Frame(type=TYPE_SYNC_DONE, id=id, payload={"cursor": cursor})
 
 
+# ---------------------------------------------------------------------------
+# Push / notification frames (M5)
+# ---------------------------------------------------------------------------
+
+def notification(
+    chat_id: str,
+    kind: str,
+    title: str,
+    body: str,
+    *,
+    thread_id: Optional[str] = None,
+    ts: Optional[int] = None,
+) -> Frame:
+    """Event: a transient in-app banner (and a push mirror when the device is
+    offline). ``kind`` is one of the ``NOTIF_*`` constants."""
+    payload: Dict[str, Any] = {"kind": kind, "title": title, "body": body}
+    if ts is not None:
+        payload["ts"] = ts
+    return Frame(
+        type=TYPE_NOTIFICATION, chat_id=chat_id, thread_id=thread_id, payload=payload
+    )
+
+
+def fcm_register(
+    fcm_token: Optional[str] = None, ntfy_topic: Optional[str] = None
+) -> Frame:
+    """Request: update the device's push tokens (FCM rotation / ntfy topic)."""
+    payload: Dict[str, Any] = {}
+    if fcm_token:
+        payload["fcm_token"] = fcm_token
+    if ntfy_topic:
+        payload["ntfy_topic"] = ntfy_topic
+    return Frame(type=TYPE_FCM_REGISTER, payload=payload)
+
+
 # ---------------------------------------------------------------------------
 # Media frames (M4)
 # ---------------------------------------------------------------------------