Add Docker setup, .env.example, updated .gitignore/CHANGELOG and ADRs 004-007 + arc42 template

.env.example

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+# Environment variables for Linear Void
+# Copy to .env and fill in real values for local/development use.
+# Production secrets should be managed via Docker secrets, Cloudflare, or Hetzner environment.
+
+# Cloudflare R2 (S3-compatible)
+R2_ACCESS_KEY_ID=your-r2-access-key
+R2_SECRET_ACCESS_KEY=your-r2-secret-key
+R2_BUCKET_NAME=linearvoid-media
+R2_ENDPOINT=https://your-account-id.r2.cloudflarestorage.com
+R2_REGION=auto
+
+# Cloudflare (for custom domain, cache rules, Workers if used)
+CLOUDFLARE_API_TOKEN=your-cloudflare-api-token
+CLOUDFLARE_ACCOUNT_ID=your-account-id
+MEDIA_DOMAIN=media.example.com
+
+# Chat / Application Database (PostgreSQL or SQLite for local)
+DATABASE_URL=postgres://user:password@localhost:5432/linearvoid
+# Or for SQLite (local development):
+# DATABASE_URL=sqlite://./data/chat.db
+
+# Redis (for presence, rate limiting, pub/sub)
+REDIS_URL=redis://localhost:6379/0
+
+# Service Ports (for local docker-compose)
+PLAYOUT_PORT=8080
+CHAT_PORT=3000
+NGINX_PORT=80
+
+# Playout configuration
+PLAYLIST_PATH=/app/playlist.txt
+HLS_SEGMENT_DURATION=6
+HLS_WINDOW_SIZE=24
+
+# Logging / Monitoring
+LOG_LEVEL=info
+PROMETHEUS_PORT=9090
+GRAFANA_PORT=3001
+
+# Development flags
+NODE_ENV=development
+GO_ENV=development