Add Docker support for local development

[SaharPak]

Context

Contributors should be able to run the bot locally with a single command, without worrying about Python version or dependencies.

What to do

1. Create a Dockerfile
2. Create a docker-compose.yml for local dev
3. Document in README how to run with Docker

Example docker-compose.yml

services:
  bot:
    build: .
    env_file: .env
    volumes:
      - ./src:/app/src
    restart: unless-stopped

Acceptance Criteria

• docker compose up starts the bot
• Hot-reload works for development (volume mount)
• README includes Docker instructions