feat(financing): competitive auction / best-offer selection helper

[samjay8]

Problem

Originators compare offers manually. A read-only best-offer helper (lowest total cost for the given duration) reduces friction and is a natural onboarding/demo feature.

Scope

• get_best_offer(invoice_id, originator) — read-only, no auth required
• Ranking: lowest total repayment (amount + interest at duration), then earliest created
• Returns offer id + computed totals; documented in the contract reference

Stop

• No automatic acceptance (originator still chooses)
• No hidden-matching/auction mechanics

Acceptance criteria

• Correct winner for a fixture with two offers at different rates
• Ties broken deterministically
• Read-only, safe for public invocation