Validate self-host deployment from a fresh Cloudflare and Modal account

[showgp]

Problem

The deployment guide has been maintained against the current implementation, but it should be validated from a fresh Cloudflare and Modal setup without private maintainer context.

Proposed Work

• Follow DEPLOYMENT.md from a new or clean account setup.
• Record any missing prerequisites, unclear environment variables, confusing names, or commands that need more context.
• Do not share real secrets, account IDs, bucket names, domains, or private documents.
• Open focused follow-up PRs or comments for each docs gap.

Acceptance Criteria

• A contributor can identify whether DEPLOYMENT.md is sufficient for a fresh self-host attempt.
• Any required docs fixes are listed clearly.
• No private account information is posted.