[codex] Fix explicit-any lint warnings

[jack-arturo]

Summary

• Replaces remaining @typescript-eslint/no-explicit-any surfaces with typed helpers and unknown-based assertions.
• Preserves the public setAPI testing hook shape while validating it internally.
• Adds a regression test for empty period validation.

Verification

• npm run lint -- --max-warnings=0
• npm run build
• npm test (55 passed, 4 skipped)
• git diff --check

Review

• Independent code-review agent initially found two Important findings.
• Both were fixed and the reviewer rechecked the final diff with no remaining Critical, Important, or Minor findings.

Stacked on top of feat/client-crud-refresh because that feature branch already has PR #61 open.