fix: read LiveKit credentials from env file instead of hardcoding

LIVEKIT_KEY/LIVEKIT_SECRET were committed in the unit since 68a6acf
(2026-06-12). That secret signs re-issued SFU join tokens, so it
controlled who could publish. Rotated and moved to /etc/matrix-deploy.env
(0600); livekit-server, lk-jwt-service and voice-limit-guard all updated
and verified against the new value.

Environment= overrides EnvironmentFile= regardless of order, so the
hardcoded lines had to be removed rather than shadowed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

systemd/voice-limit-guard.service

@@ -8,15 +8,17 @@ Type=simple
 ExecStart=/usr/bin/env python3 /opt/voice-limit-guard/voice-limit-guard.py
 Restart=on-failure
 RestartSec=5
-# MATRIX_TOKEN (server-admin) is read from the existing deploy env file.
+# Secrets come from the deploy env file, never from this unit — it is in git.
+# Provides MATRIX_TOKEN (server-admin), LIVEKIT_KEY and LIVEKIT_SECRET.
+# NOTE: LIVEKIT_KEY/SECRET must match /etc/livekit/config.yaml and
+# lk-jwt-service (which reads the same env file). Rotating means changing
+# all three together, or every call fails to get a token.
 EnvironmentFile=/etc/matrix-deploy.env
 Environment=GUARD_BIND_HOST=0.0.0.0
 Environment=GUARD_BIND_PORT=8070
 Environment=GUARD_UPSTREAM=http://127.0.0.1:8071
 Environment=LIVEKIT_API=http://127.0.0.1:7880
 Environment=SYNAPSE_API=http://127.0.0.1:8008
-Environment=LIVEKIT_KEY=lotuskey
-Environment=LIVEKIT_SECRET=GoI5PPLbNXZlQHlfdAzLFy0B/QoqA9uXiyb/p6dQEtc=
 
 [Install]
 WantedBy=multi-user.target