feat: tunnel JWT auth

Tunnel method now requires Bearer JWT (HMAC-HS256) in gRPC metadata.
   - internal/auth: pure-stdlib verifier + issuer + revoked-jti list
   - internal/tunnel: per-target rate limiter + egress allowlist with
     wildcard support + crypto/rand frame padding to {1KB, 4KB, 16KB}
   - cmd/server reads /etc/stats-gateway/{jwt.key,tunnel.yaml,revoked.txt}
     via --config-dir flag
   - cmd/issue-token Go binary wraps auth.Issue
   - configs/tunnel.yaml example with projectshitpost wildcard


Tests: 13 PASS across 5 packages.

Makefile

@@ -4,6 +4,7 @@ build:
 	go build -o bin/stats-gateway ./cmd/server
 	go build -o bin/stats-collector ./cmd/collector
 	go build -o bin/stats-snapshot ./cmd/snapshot
+	go build -o bin/stats-issue-token ./cmd/issue-token
 
 test:
 	go test ./...