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.

go.sum

@@ -1,3 +1,7 @@
+github.com/MicahParks/jwkset v0.11.1 h1:Gr+6JeYoJ7twqyBGf8MXxj2NSCKPoX8RFMtey3GY9Nw=
+github.com/MicahParks/jwkset v0.11.1/go.mod h1:U2oRhRaLgDCLjtpGL2GseNKGmZtLs/3O7p+OZaL5vo0=
+github.com/MicahParks/keyfunc/v3 v3.8.1 h1:VR3jlEs2wz1xGjvUUwUHeoB+eJ6gUnBrpnIByRrtj6g=
+github.com/MicahParks/keyfunc/v3 v3.8.1/go.mod h1:LcorJ0sz2tZGvgZqIfaeyLkJmM+kxIfRDu8dFY0TAas=
 github.com/VictoriaMetrics/metrics v1.44.0 h1:Fr8yqQSV+ZfYaDD/anqk1E8e9YPgfleSleJmAI0M0Tw=
 github.com/VictoriaMetrics/metrics v1.44.0/go.mod h1:xDM82ULLYCYdFRgQ2JBxi8Uf1+8En1So9YUwlGTOqTc=
 github.com/alicebob/gopher-json v0.0.0-20200520072559-a9ecdc9d1d3a h1:HbKu58rmZpUGpz5+4FfNmIU+FmZg2P3Xaj2v2bfNWmk=
@@ -42,6 +46,8 @@ github.com/gofiber/utils/v2 v2.0.6 h1:7fXYy7nSsyqbH0GQUMtK4Kwjy4J7R5742VM7JsZxzO
 github.com/gofiber/utils/v2 v2.0.6/go.mod h1:p7mAHAk3+oUK10ZX2xTw9fZQixb4hCg8SKd4IH2xroU=
 github.com/gofrs/flock v0.13.0 h1:95JolYOvGMqeH31+FC7D2+uULf6mG61mEZ/A8dRYMzw=
 github.com/gofrs/flock v0.13.0/go.mod h1:jxeyy9R1auM5S6JYDBhDt+E2TCo7DkratH4Pgi8P+Z0=
+github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
+github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
 github.com/golang/protobuf v1.5.4 h1:i7eJL8qZTpSEXOPTxNKhASYpMn+8e5Q6AdndVa1dWek=
 github.com/golang/protobuf v1.5.4/go.mod h1:lnTiLA8Wa4RWRcIUkrtSVa5nRhsEGBg48fD6rSs7xps=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
@@ -160,6 +166,8 @@ golang.org/x/term v0.44.0 h1:0rLvDRCtNj0gZkyIXhCyOb2OAzEhLVqc4B+hrsBhrmc=
 golang.org/x/term v0.44.0/go.mod h1:7ze4MdzUzLXpSAoFP1H0bOI9aXDqveSvatT5vKcFh2Y=
 golang.org/x/text v0.39.0 h1:UbZz4pLOvn600D6Oh6GGEI6VAmndrEBLv8/6BEXzyus=
 golang.org/x/text v0.39.0/go.mod h1:3UwRclnC2g0TU9x8PZiyfOajCd1zaUNHF9cvqcQZ+ZM=
+golang.org/x/time v0.15.0 h1:bbrp8t3bGUeFOx08pvsMYRTCVSMk89u4tKbNOZbp88U=
+golang.org/x/time v0.15.0/go.mod h1:Y4YMaQmXwGQZoFaVFk4YpCt4FLQMYKZe9oeV/f4MSno=
 gonum.org/v1/gonum v0.17.0 h1:VbpOemQlsSMrYmn7T2OUvQ4dqxQXU+ouZFQsZOx50z4=
 gonum.org/v1/gonum v0.17.0/go.mod h1:El3tOrEuMpv2UdMrbNlKEh9vd86bmQ6vqIcDwxEOc1E=
 google.golang.org/genproto/googleapis/api v0.0.0-20260618152121-87f3d3e198d3 h1:ctPmKL12ZsoKAlmPUsoW70zEDiYF+/H6aLieXxgAU0k=