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.

proxy.go

@@ -713,7 +713,14 @@ func performProxyRequest(c fiber.Ctx, proxyURL string) error {
 	// reached without going through handleCaching (direct callers).
 	cacheKey, _ := c.Locals("query_cache_hash").(string)
 	if cacheKey == "" {
-		userID, userRole := extractUserInfo(c)
+		// Error intentionally ignored: the primary rejection for an
+		// invalid/forged token already happened in processGraphQLRequest
+		// (it 401s before the request ever reaches here). On a verify
+		// failure extractUserInfo returns ("-", "-", err), so a forged
+		// token still only maps into the shared anonymous cache bucket
+		// below, never into a genuine, verified user's own bucket
+		// (GHSA-9gqw-h2rw-44wv).
+		userID, userRole, _ := extractUserInfo(c)
 		cacheKey = libpack_cache.CalculateHash(c, userID, userRole)
 	}