test(net): TLS-enabled docker stack and active e2e test

The docker-compose stack now serves Headscale (and its embedded DERP)
over TLS on port 8443 with a self-signed cert covering localhost,
127.0.0.1, and the docker-network hostname `headscale`. Tailscale
peers trust the cert via SSL_CERT_FILE; our test daemon uses
`derp_tls_insecure: true` (gated on the SUNBEAM_NET_TEST_DERP_INSECURE
env var) since pinning a self-signed root in tests is more trouble
than it's worth.

With TLS DERP working, the previously-ignored
`test_e2e_tcp_through_tunnel` test now passes: the daemon spawns,
registers, completes a Noise handshake over TLS, opens a TLS DERP
relay session, runs a real WireGuard handshake with peer-a (verified
via boringtun ↔ tailscale interop), and TCP-tunnels an HTTP GET
through smoltcp ↔ engine ↔ proxy ↔ test client. The 191-byte echo
response round-trips and the test asserts on its body.

- tests/config/headscale.yaml: tls_cert_path + tls_key_path, listen on
  8443, server_url=https://headscale:8443
- tests/config/test-cert.pem + test-key.pem: 365-day self-signed RSA
  cert with SAN DNS:localhost, DNS:headscale, IP:127.0.0.1
- tests/docker-compose.yml: mount certs into headscale + both peers,
  set SSL_CERT_FILE on the peers, expose 8443 instead of 8080
- tests/run.sh: switch to https://localhost:8443, set
  SUNBEAM_NET_TEST_DERP_INSECURE=1
- tests/integration.rs: drop the #[ignore] on test_e2e_tcp_through_tunnel,
  read derp_tls_insecure from env in all four test configs

sunbeam-net/tests/run.sh

@@ -59,7 +59,8 @@ $COMPOSE exec -T peer-b tailscale ip -4 || true
 echo "==> Running integration tests..."
 cd ../..
 SUNBEAM_NET_TEST_AUTH_KEY="$CLIENT_KEY" \
-SUNBEAM_NET_TEST_COORD_URL="http://localhost:8080" \
+SUNBEAM_NET_TEST_COORD_URL="https://localhost:8443" \
+SUNBEAM_NET_TEST_DERP_INSECURE=1 \
 SUNBEAM_NET_TEST_PEER_A_IP=$($COMPOSE -f sunbeam-net/tests/docker-compose.yml exec -T peer-a tailscale ip -4 2>/dev/null | tr -d '[:space:]') \
   cargo test -p sunbeam-net --features integration --test integration -- --nocapture