Add setup and deployment guide, env example files

- Rewrote README.md with full step-by-step setup guide covering Docker
  Compose deployment, environment configuration, port reference, local
  dev setup, and reset instructions
- Added backend/.env.example and frontend/.env.local.example so new
  deployers know exactly what to configure without any secrets committed
- Updated .gitignore to allow .env.example files through

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/.env.example

@@ -0,0 +1,24 @@
+# ─────────────────────────────────────────────────────────────────
+# BMS Backend — Environment Variables
+# Copy this file to .env and fill in your values.
+# Never commit .env to git.
+# ─────────────────────────────────────────────────────────────────
+
+# Database — matches docker-compose.yml defaults, no change needed for Docker deploys
+DATABASE_URL=postgresql+asyncpg://dcim:dcim_pass@db:5432/dcim
+
+# MQTT broker — matches the mqtt service name in docker-compose.yml
+MQTT_HOST=mqtt
+MQTT_PORT=1883
+
+# Clerk authentication
+# Get these from https://dashboard.clerk.com → Your App → API Keys
+CLERK_SECRET_KEY=sk_test_REPLACE_ME
+CLERK_JWKS_URL=https://YOUR_APP.clerk.accounts.dev/.well-known/jwks.json
+
+# CORS origins — only needed if the backend is accessed directly from a browser
+# Example: CORS_ORIGINS=["https://yourdomain.com"]
+# Leave empty when using the built-in Next.js proxy (recommended)
+CORS_ORIGINS=[]
+
+DEBUG=true