feat: DigitalOcean deployment support with .do config and Deploy button

[open-swe]

Fixes #48

What this PR adds:

• New .do directory at the repo root for DigitalOcean App Platform configuration.
• .do/app.yaml for the Go service built from the existing Dockerfile, including environment variable placeholders marked as SECRET for sensitive values (MongoDB, Redis, JWT, S3/MinIO, OAuth credentials, and SMTP).
• .do/deploy.template.yaml enabling the Deploy to DigitalOcean button, referencing the tryanzu/anzu GitHub repository, building from Dockerfile, and configuring environment variables with DO connections to MongoDB and Redis.
• README.md updated with a Deploy to DigitalOcean button and a deployment guide detailing required and optional environment variables for App Platform configuration.

How to deploy:

• Click the DigitalOcean Deploy button to start the deployment flow.
• In DigitalOcean App Platform, connect to GitHub (tryanzu/anzu) and set up the app using the provided templates.
• In App Platform settings, configure environment variables as described in the deployment guide, marking sensitive values as SECRET.

Environment variables (examples and placeholders):

• MongoDB/MongoDB URI and related connection details
• Redis connection details
• JWT_SECRET (SECRET)
• S3/MinIO credentials (S3_ENDPOINT, S3_KEY, S3_SECRET, S3_BUCKET) marked as SECRET
• OAuth credentials (e.g., GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET) marked as SECRET
• SMTP settings (SMTP_HOST, SMTP_PORT, SMTP_USER, SMTP_PASSWORD) with SMTP_PASSWORD marked as SECRET

Notes:

• Adjust variable names to your actual deployment needs and ensure DO services are accessible from App Platform.
• This PR provides the scaffolding; you will need to fill in the placeholder SECRET values after deployment.