fix: pin JWT algorithm to HS256 and harden token security

- Add { algorithms: ['HS256'] } to all jwt.verify() calls to prevent
  algorithm confusion attacks (including the 'none' algorithm)
- Add { algorithm: 'HS256' } to all jwt.sign() calls for consistency
- Reduce OIDC token payload to only { id } (was leaking username, email, role)
- Validate OIDC redirect URI against APP_URL env var when configured
- Add startup warning when JWT_SECRET is auto-generated

https://claude.ai/code/session_01SoQKcF5Rz9Y8Nzo4PzkxY8

server/src/config.ts

@@ -23,4 +23,6 @@ if (!JWT_SECRET) {
   }
 }
 
-export { JWT_SECRET };
+const JWT_SECRET_IS_GENERATED = !process.env.JWT_SECRET;
+
+export { JWT_SECRET, JWT_SECRET_IS_GENERATED };