feat: harden premium payment date progression logic

[Mosas2000]

Implement deterministic next_payment_date updates to prevent schedule drift:

• Add PREMIUM_INTERVAL_SECONDS constant (30 days) for consistent interval usage
• Modify pay_premium to advance from previous due date, not current timestamp
• Handle late payments by advancing until next_due is in the future
• Prevent schedule manipulation through early or late payment timing

The new logic ensures:

• Early payments don't shift the schedule forward
• Late payments don't double-cover periods
• Payment schedules remain predictable regardless of payment timing

Test coverage includes:

• Early payment scenarios (5-15 days early)
• On-time payment boundary cases
• Late payment single/multiple period scenarios
• Mixed early/late payment sequences over 12 months
• Boundary condition at exactly next_due timestamp

Resolves: premium payment date drift issue

Closes #278