Add docker setup and switch voice chat backend to use livekit

.env.example

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+# Copy this file to .env and fill in the values before running docker compose.
+
+# Required: a long random secret used for auth token signing.
+# Generate one with: openssl rand -hex 32
+CHGRID_AUTH_SECRET=
+
+# The URL the browser uses to reach the app (no trailing slash).
+# Default is http://localhost for local use.
+# Set to your public URL for production, e.g. https://example.com
+CHGRID_HOST_ORIGIN=http://localhost
+
+# LiveKit SFU settings (required for voice audio).
+# These must match the keys configured in livekit.yaml.
+LIVEKIT_API_KEY=your-api-key
+LIVEKIT_API_SECRET=your-api-secret
+# This URL is sent to browsers, so it must be reachable from the client (not Docker-internal).
+LIVEKIT_URL=ws://localhost:7880