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.

struct_config.go

@@ -42,21 +42,26 @@ type config struct {
 		PerUserCacheDisabled  bool // Disable per-user cache isolation (SECURITY RISK - not recommended)
 	}
 	Client struct {
-		GQLClient            *graphql.BaseClient
-		FastProxyClient      *fasthttp.Client
-		JWTUserClaimPath     string
-		JWTRoleClaimPath     string
-		RoleFromHeader       string
-		proxy                string
-		ClientTimeout        int
-		MaxConnsPerHost      int
-		ReadTimeout          int
-		WriteTimeout         int
-		MaxIdleConnDuration  int
-		RetryBaseDelayMs     int // initial backoff delay in milliseconds (default 500)
-		RetryMaxDelayMs      int // maximum backoff delay in milliseconds (default 10000)
-		RoleRateLimit        bool
-		DisableTLSVerify     bool
+		GQLClient        *graphql.BaseClient
+		FastProxyClient  *fasthttp.Client
+		JWTUserClaimPath string
+		JWTRoleClaimPath string
+		// JWTVerifier verifies JWT signatures before extractClaimsFromJWTHeader
+		// trusts a claim from the token (GHSA-9gqw-h2rw-44wv). nil (the default)
+		// means JWT_VERIFY_SIGNATURE is off and claims are decoded unverified,
+		// exactly like before this field existed.
+		JWTVerifier         *JWTVerifier
+		RoleFromHeader      string
+		proxy               string
+		ClientTimeout       int
+		MaxConnsPerHost     int
+		ReadTimeout         int
+		WriteTimeout        int
+		MaxIdleConnDuration int
+		RetryBaseDelayMs    int // initial backoff delay in milliseconds (default 500)
+		RetryMaxDelayMs     int // maximum backoff delay in milliseconds (default 10000)
+		RoleRateLimit       bool
+		DisableTLSVerify    bool
 	}
 	Server struct {
 		HostGraphQL         string