[Contracts] RemittanceNFT decrease_score and set_min_repayment_amount panic instead of returning typed NftError

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

remittance_nft/src/lib.rs:715-721 decrease_score returns () and panics via unwrap_or_else(|_| panic!(unauthorized minter)) / expect(user does not have an NFT), and set_min_repayment_amount (693-697) does panic!(negative amount). Every sibling mutator (mint, update_score, seize_collateral, burn, transfer, apply_score_delta) returns Result<(), NftError> with typed variants, so these two break the contract's error API and give callers no structured error (cross-contract callers can only observe a generic trap).

Acceptance criteria

• decrease_score returns Result<(), NftError> using UnauthorizedMinter and NftNotFound instead of panicking
• set_min_repayment_amount returns an error variant (e.g. a new InvalidAmount/BelowMinimum) for negative input instead of panic!
• Existing should_panic tests updated to assert the typed errors

Files to touch

• contracts/remittance_nft/src/lib.rs

Out of scope

• Changing decrease_score scoring/floor logic (tracked separately)
• Altering MultisigGovernance error handling