feat(mobile): per-user device bindings, multi-user Inbox routing, and admin-mediated authorization

- Device→user bindings persisted in config table (auth.rs), loaded at plugin start
- RelayApp now routes Inbox responses per-user via UserChannelApi, never globally
- New mobile_bind_device LLM tool for admin-mediated device→user assignment
- Per-user event forwarders (events.rs) with per-user debounced notifiers
- Config listener (auth::config_listener) refreshes bindings cache reactively
- Reconcile loop catches users who unlock after boot
- Hello/Logout treated as device-registry ops (no user resolution needed)
- Unbound device payloads are silently dropped
- RelayAgent::authorize_client → bind_device (atomic bind + authorize)
- Approval rules seed mobile_bind_device/revoke_device as require

crates/skald-core/src/skald/user_context.rs

@@ -32,6 +32,7 @@ use tokio_util::sync::CancellationToken;
 use core_api::approval::ApprovalApi;
 use core_api::chat_hub::ChatHubApi;
 use core_api::events::GlobalEvent;
+use core_api::inbox::InboxApi;
 use core_api::system_bus::SystemEventBus;
 use core_api::user_channel::UserChannelHandle;
 
@@ -292,6 +293,12 @@ impl UserChannelHandle for UserContextHandle {
         Arc::clone(&self.ctx.approval) as Arc<dyn ApprovalApi>
     }
 
+    fn inbox(&self) -> Arc<dyn InboxApi> {
+        // `Inbox` is a cheap facade over the interaction-stack `Arc`s (Clone);
+        // the clone shares the same pending state as the context's own inbox.
+        Arc::new(self.ctx.inbox.clone()) as Arc<dyn InboxApi>
+    }
+
     fn subscribe(&self) -> broadcast::Receiver<GlobalEvent> {
         self.ctx.global_tx.subscribe()
     }