[User Story] Add OpenTofu module variables for Ghost config

[noahwhite]

Story Summary

As a platform engineer, I want to add OpenTofu variables for the non-secret Ghost configuration, so that domain names, paths, and mail settings can be passed to the instance module.

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✅ Acceptance Criteria

• ghost_domain variable added (string, required) - Ghost site domain
• ghost_admin_domain variable added (string, required) - Ghost admin panel domain
• ghost_version variable added (string, default "6-alpine") - Ghost Docker image tag
• mail_smtp_host variable added (string, default "smtp.mailgun.org") - SMTP host
• mail_smtp_user variable added (string, required) - SMTP username
• admin_ip variable added (string, required) - Admin workstation IP for Caddy ACL
• Variables documented with descriptions
• No sensitive values accepted (passwords, tokens)

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📝 Additional Context

• Design: These variables are non-secret configuration that's safe to pass through OpenTofu
• Secrets (DATABASE_PASSWORD, HEALTH_CHECK_TOKEN, etc.) remain on block storage
• Some variables may already exist (ghost_url) and can be reused or renamed

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📦 Definition of Ready

• Acceptance criteria defined
• No unresolved external dependencies
• Story is estimated
• Team has necessary skills and access
• Priority is clear
• Business value understood

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✅ Definition of Done

• All acceptance criteria met
• OpenTofu syntax valid (tofu validate passes)
• Peer-reviewed
• Variables properly typed and documented

[linear]

GHO-49