feat(webui): auth and deployment boundary (#435)

Add bind-host assessment that refuses 0.0.0.0/:: without override,
warns on non-loopback binds, and documents internal-only MVP serving.
Health endpoint exposes deployment metadata; docs cover Access/VPN/WARP
and runtime env assumptions without embedding secrets in the client.

Closes #435

webui/app.py

@@ -9,6 +9,7 @@ from starlette.requests import Request
 from starlette.responses import HTMLResponse, JSONResponse, Response
 from starlette.routing import Route
 
+from webui.deployment_boundary import deployment_snapshot
 from webui.layout import render_page
 from webui.project_registry import find_project, load_registry, registry_to_dict
 from webui.project_views import render_project_detail, render_projects_list
@@ -47,11 +48,13 @@ async def home(_request: Request) -> HTMLResponse:
 
 
 async def health(_request: Request) -> JSONResponse:
+    bind_host = _request.app.state.webui_bind_host
     return JSONResponse({
         "status": "ok",
         "service": "mcp-control-plane-webui",
         "mode": "read-only-mvp",
         "timestamp": datetime.now(timezone.utc).isoformat(),
+        "deployment": deployment_snapshot(bind_host=bind_host),
     })
 
 
@@ -156,9 +159,12 @@ async def method_not_allowed(request: Request, _exc: Exception) -> Response:
     return JSONResponse({"error": "not_found"}, status_code=404)
 
 
-def create_app() -> Starlette:
+def create_app(*, bind_host: str | None = None) -> Starlette:
     """Build the read-only MVP Starlette app."""
-    return Starlette(
+    import os
+
+    resolved_bind = bind_host or os.environ.get("WEBUI_HOST", "127.0.0.1")
+    app = Starlette(
         debug=False,
         routes=[
             Route("/", home, methods=["GET"]),
@@ -177,4 +183,6 @@ def create_app() -> Starlette:
             Route("/leases", leases, methods=["GET"]),
         ],
         exception_handlers={405: method_not_allowed},
-    )
+    )
+    app.state.webui_bind_host = resolved_bind
+    return app