refactor: revert fly.ai.toml rename, keep fly.toml for single app

[devin-ai-integration]

Summary

Reverts the unnecessary fly.toml → fly.ai.toml rename introduced in #3852. Since we're keeping a single Fly app (with multiple domains via flyctl certs), there's no need for a separate config filename — fly.toml is sufficient.

• Renames apps/api/fly.ai.toml back to apps/api/fly.toml
• Updates api_cd.yaml deploy command to reference fly.toml

Review & Testing Checklist for Human

• Confirm no other files reference fly.ai.toml (should be clean since it was only introduced in refactor: consolidate apps/ai into apps/api #3852)

Notes

Requested by @yujonglee | Devin session

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✅ Devin Review: No Issues Found

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