feat: extend /api/instance-info to return device_id

Add load_device_id import from muxplex.identity and include device_id
in the instance-info response dict. Update endpoint docstring to
mention device identity.

Test: test_instance_info_includes_device_id verifies device_id is
present as a non-empty string when IDENTITY_PATH is redirected to
tmp_path.

muxplex/main.py

@@ -71,6 +71,7 @@ from muxplex.settings import (
     load_settings,
     patch_settings,
 )
+from muxplex.identity import load_device_id
 from muxplex.ttyd import kill_orphan_ttyd, kill_ttyd, spawn_ttyd, TTYD_PORT
 
 # ---------------------------------------------------------------------------
@@ -865,16 +866,17 @@ async def put_settings_sync(payload: SettingsSyncPayload):
 
 @app.get("/api/instance-info")
 async def instance_info() -> dict:
-    """Return this instance's display name and version.
+    """Return this instance's display name, device identity, and version.
 
     Public endpoint (no auth required) — used by remote instances to
-    discover peer names and verify reachability.
+    discover peer names, device identity, and verify reachability.
     """
     settings = load_settings()
     # Read fresh so the UI reflects key-file changes without requiring a restart.
     fed_key = load_federation_key()
     return {
         "name": settings["device_name"],
+        "device_id": load_device_id(),
         "version": app.version,
         "federation_enabled": bool(fed_key),
     }

muxplex/tests/test_api.py

@@ -1294,6 +1294,26 @@ def test_instance_info_federation_enabled_true_when_key_exists(
     )
 
 
+def test_instance_info_includes_device_id(client, tmp_path, monkeypatch):
+    """GET /api/instance-info includes device_id as a non-empty string."""
+    import muxplex.identity as identity_mod
+    import muxplex.settings as settings_mod
+
+    monkeypatch.setattr(settings_mod, "SETTINGS_PATH", tmp_path / "settings.json")
+    monkeypatch.setattr(identity_mod, "IDENTITY_PATH", tmp_path / "identity.json")
+
+    response = client.get("/api/instance-info")
+    assert response.status_code == 200
+    data = response.json()
+    assert "device_id" in data, f"Response must include 'device_id' key, got: {data}"
+    assert isinstance(data["device_id"], str), (
+        f"device_id must be a string, got: {type(data['device_id'])}"
+    )
+    assert data["device_id"] != "", (
+        f"device_id must be a non-empty string, got: {data['device_id']!r}"
+    )
+
+
 # ---------------------------------------------------------------------------
 # POST /api/sessions (create new session)
 # ---------------------------------------------------------------------------