The proxy derived per-user GraphQL cache keys from JWT claims without
verifying the token signature (CWE-345). A remote attacker could forge a
token that carried a victim's claim value and read the victim's cached
response.
This change adds opt-in signature verification behind JWT_VERIFY_SIGNATURE.
The flag defaults to false, so an upgrade without a config change keeps the
previous delegate-to-upstream behavior byte for byte.
When verification is on, the operator must set exactly one key source:
JWT_SECRET, JWT_PUBLIC_KEY, or JWT_JWKS_URL. The verifier pins the accepted
algorithms to the key family through WithValidMethods and a type-switched
keyfunc, and it refuses alg=none. This closes the algorithm-confusion class
of attack. A present but invalid token now returns 401. A missing
Authorization header stays anonymous.
Startup fails loudly when verification is on and the key source is absent or
unreadable, so the proxy can never boot half-configured.
