docs: add .env.example documenting backend environment variables

[birme]

Summary

• Add .env.example at the backend repo root documenting every environment variable the code actually reads (discovered via grep process.env in src/).
• Variables are grouped into Required (SMB_ADDRESS) and Optional (with defaults shown), each with a short comment and a placeholder example value — no real secrets.
• Covers DB connection (DB_CONNECTION_STRING / legacy MONGODB_CONNECTION_STRING), server (PORT, PUBLIC_HOST, ENDPOINT_IDLE_TIMEOUT_S), SMB (SMB_APIKEY), ICE/CORS, WHIP lifecycle tuning, OSC integration, and DEBUG.
• Add explicit !.env.example negation to .gitignore so the example stays tracked while real .env remains ignored.

Test plan

• npm install && npm run lint — 0 errors (pre-existing warnings only).
• git status confirms .env.example is staged/tracked.
• No source code changed; package-lock.json churn reverted.

Closes #275