[User Story] Update create-storage-dirs.service for secrets directory

[noahwhite]

Story Summary

As a platform engineer, I want to update create-storage-dirs.service to create the ghost-compose secrets directory on block storage, so that .env.secrets has a proper location with secure permissions.

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✅ Acceptance Criteria

• Service creates /var/mnt/storage/ghost-compose directory
• Directory has mode 0700 (owner-only access)
• Existing directories for MySQL, Ghost content, Caddy certs remain unchanged
• Service unit syntax valid

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📝 Additional Context

• Design: This directory only holds .env.secrets - the rest of compose config is in /etc/ghost-compose/
• The 0700 permission is more restrictive than other directories since it contains secrets
• The .env.secrets file itself should be 0600 (created manually)

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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
• Peer-reviewed
• Systemd unit syntax valid

[linear]

GHO-52