fix(auth): verify JWT signature to prevent cache-key forgery (GHSA-9gqw-h2rw-44wv)

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.

details_test.go

@@ -73,7 +73,8 @@ func (suite *Tests) Test_extractClaimsFromJWTHeader() {
 			if len(tt.jwt_role_path) > 0 {
 				cfg.Client.JWTRoleClaimPath = tt.jwt_role_path
 			}
-			gotUsr, gotRole := extractClaimsFromJWTHeader(tt.args.authorization)
+			gotUsr, gotRole, err := extractClaimsFromJWTHeader(tt.args.authorization)
+			suite.NoError(err, "Unexpected verification error")
 			suite.Equal(tt.wantUsr, gotUsr, "Unexpected user ID")
 			suite.Equal(tt.wantRole, gotRole, "Unexpected role")
 		})