feat: add docker-compose with LiveKit for server Docker deployment

- docker-compose.yml: owncord + livekit/livekit-server on shared network
- Secrets (API key/secret) injected via .env → OWNCORD_VOICE_* env vars
- livekit.yaml.example: template config with port ranges and node_ip guidance
- .env.example: secret template with min-length reminder for API secret
- .gitignore: add .env / Server/.env to prevent accidental secret commits

Users: cp .env.example .env && cp livekit.yaml.example livekit.yaml,
fill in values, then docker compose up -d

.gitignore

@@ -1,3 +1,7 @@
+# Docker secrets
+.env
+Server/.env
+
 # Claude Code local config
 .claude/
 Client/.claude/