Add userId field to McpInstallation interface

- Add userId: string field to distinguish individual users from OAuth clients
- Prepares for user session isolation implementation
- clientId represents OAuth application, userId represents individual user

This separation allows multiple users to use the same OAuth client while
maintaining session isolation for security.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/types.ts

@@ -27,4 +27,5 @@ export interface McpInstallation {
   mcpTokens: OAuthTokens;
   clientId: string;
   issuedAt: number;
+  userId: string; // Unique identifier for the user (not client)
 }