feat(server): add Bun HTTP server with webhook endpoint
Implements src/index.ts (bead assistant-pqh.1, assistant-pqh.3)
Server features:
- Bun.serve() with fetch handler pattern
- GET /health - full dependency health check
- GET /healthz - simple liveness probe (k8s)
- POST /webhook - Telegram webhook endpoint
Webhook security:
- Validates X-Telegram-Bot-Api-Secret-Token header
- Returns 401 for missing/invalid tokens
- Parses Telegram Update JSON body
- Fire-and-forget pattern for fast 200 OK response
Startup:
- Initializes Letta before accepting requests
- Starts polling mode when TELEGRAM_WEBHOOK_URL empty
- Graceful error handling for initialization failures
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>