feat(repayment): fixed installment repayment schedules

[samjay8]

Problem

Repayment is currently open (any amount, any time) until fully cleared. Businesses that prefer fixed installments (weekly/monthly) have no structure, and lenders can't forecast cash flows.

Scope

• ADR note on schedule model (equal principal + interest on remaining, or amortized)
• schedule_repayment(offer_id, frequency, count) at financing/repayment boundary — Pending/Financed offers
• Keeper/CLI-friendly 'due installment' read helper
• Partial repayments still allowed; schedule is advisory-with-enforcement option (documented)

Stop

• No penalty logic for missed installments (that's future work)
• No compounding schedules

Acceptance criteria

• Schedule created and readable
• Installment math matches the documented model at a known rate
• Off-schedule payment still permitted without corrupting state