feat(mcp): per-request user_id contextvar for tool handlers

Adds mcp._context.current_user_id() backed by a ContextVar. The ASGI
auth middleware sets it before dispatching to FastMCP and resets it
on the way out, so tool handlers can read the acting user without
re-parsing the request scope.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/server.py

@@ -63,7 +63,12 @@ def mount_mcp(app: Quart) -> None:
             })
             return
         scope["fable_user_id"] = user_id
-        await mcp_asgi(scope, receive, send)
+        from fabledassistant.mcp._context import _user_id_ctx
+        token = _user_id_ctx.set(user_id)
+        try:
+            await mcp_asgi(scope, receive, send)
+        finally:
+            _user_id_ctx.reset(token)
 
     original_asgi = app.asgi_app