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.

configs/tunnel.yaml

@@ -0,0 +1,6 @@
+egress:
+  allowed:
+    - "192.0.2.10:8443"          # FL xray inbound (production)
+    - "*.projectshitpost.fun:443" # any subdomain HTTPS
+    # TODO: add PL1 IP:port once confirmed (controller to fill in during deploy)
+  rate_mbps: 50