`MIN_PAYMENT_AMOUNT = 100` is a magic number without rationale

[therealjhay]

ProblemThe minimum payment constant (lib.rs:162) has no doc explaining its derivation or policy; changing it silently changes payment viability.

1. No rationale comment.
2. Magic number.
3. Policy opacity.

Solution

• Document the rationale and add a constant test.

Acceptance Criteria

• Rationale documented.
• Constant tested.
• cargo test --workspace passes.

Note for Contributors: Write a clear PR description. Show the rationale doc.

[RajeshRk18]

I can work on this. Please assign this to me!

[grantfox-oss]

🦊 GrantFox — @RajeshRk18 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #577)
2. Your PR will be reviewed by the Betta-Pay maintainers

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