Deploy bot to AWS Lambda with webhook mode

[SaharPak]

Context

The bot currently runs in polling mode (good for dev). For production, webhook mode on serverless is more reliable and cost-effective.

Requirements

• Add webhook handler in src/main.py (AWS Lambda entry point)
• Create infra/ directory with deployment config (SAM template or Terraform)
• Set up Telegram webhook URL via Bot API
• Add deployment instructions to README
• Environment variables managed via AWS Secrets Manager or Lambda env vars

Acceptance Criteria

• Bot runs on AWS Lambda triggered by API Gateway
• Webhook is auto-registered on deploy
• Deployment is one command (sam deploy or similar)
• README has clear deployment guide