fix: unused import PAYMENT_TOLERANCE in payment_link.rs causes Clippy CI failure

[Pvsaint]

Problem

payment_link.rs line 6 imports crate::PAYMENT_TOLERANCE but the symbol is never referenced in the file:

warning: unused import: `crate::PAYMENT_TOLERANCE`
 --> fluxapay/src/payment_link.rs:6:5

CI enforces -D warnings so this blocks the pipeline.

Acceptance Criteria

• The unused import is removed from payment_link.rs
• If PAYMENT_TOLERANCE is actually needed for link validation (e.g., in use_link), it is used correctly rather than simply removed
• cargo clippy --all-targets --all-features -- -D warnings passes

Branch & Commit Examples

branch: fix/clippy-warnings-cleanup
commit: fix(payment-link): remove unused PAYMENT_TOLERANCE import