test: add unit tests for money, strkey, retention, supervise (#436 #437 #438 #439)

[Young850]

What and why

Closes #436
Closes #437
Closes #438
Closes #439

How it was verified

• cargo test passes
• cargo fmt --check and cargo clippy --all-targets -- -D warnings pass
• New behaviour is covered by a test that fails without this change

Risk

• Adds or changes a database migration (forward-only — see DEPLOYMENT.md)
• Changes a public API response shape or error code
• Touches payment verification, settlement, or amount handling
• Touches authentication, the SSRF guard, or webhook signing
• Adds or updates a dependency (cargo deny check all passes)
• Requires a config/env change to deploy (documented in README + .env.example)

Notes for the reviewer

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