Add QR pairing (terminal QR, in-app scanner, iris://pair deep link)

docs/08-push.md

@@ -1,7 +1,7 @@
 # 08 — Push Notifications, Outbox & Sync
 
 The gateway can't reach a sleeping phone directly. Push goes through a cloud
-relay. **Decision: FCM primary, ntfy fallback** (`ANDROID_PUSH_BACKEND`).
+relay. **Decision: FCM primary, ntfy fallback** (`IRIS_PUSH_BACKEND`).
 
 ## 8.1 When push fires
 
@@ -23,13 +23,13 @@ class PushBackend(Protocol):
     def configured(self) -> bool: ...
 ```
 
-Selected at adapter init by `ANDROID_PUSH_BACKEND` (`fcm` default, `ntfy`).
+Selected at adapter init by `IRIS_PUSH_BACKEND` (`fcm` default, `ntfy`).
 
 ### 8.2.1 `FcmBackend` (primary)
 - **FCM HTTP v1 API** via `httpx` (core dep). Auth = Firebase **service
-  account** (`ANDROID_FCM_SERVICE_ACCOUNT` JSON path) → mint a short-lived
+  account** (`IRIS_FCM_SERVICE_ACCOUNT` JSON path) → mint a short-lived
   OAuth2 access token (cached, refreshed before expiry). 
-- Fallback: legacy **server key** (`ANDROID_FCM_SERVER_KEY`) if no service
+- Fallback: legacy **server key** (`IRIS_FCM_SERVER_KEY`) if no service
   account (simpler, but legacy).
 - Target = the device's **FCM token** (registered via `hello` /
   `fcm.register`, stored in `devices.db`).