feat: implement subscription payment retry with intelligent scheduling (#964)

- Add IntelligentRetryScheduler with exponential backoff, jitter, and
  historical success-time shifting (moves retries toward UTC hours that
  historically succeed for each subscriber)
- Add per-plan RetryPolicy with configurable maxRetries, baseDelayHours,
  backoffMultiplier, maxDelayHours, circuitBreakerThreshold, and jitterHours
- Add circuit-breaker that pauses retries after N consecutive failures and
  auto-resets after a configurable cooldown window
- Classify all 8 decline codes (insufficient_funds, card_expired,
  card_lost_stolen, do_not_honor, authentication_required, generic_decline,
  network_error, processing_error) with code-specific base delays and
  outreach channels
- Trigger card-updater flow on card_expired (once per invoice)
- Suggest automatic 50% amount split for charges above $500 on first retry
- Add DunningOrchestrator wiring scheduler + grace-period management +
  injectable OutreachDispatcher
- Grace periods: configurable duration, per-interval email reminders,
  sweep-expired helper for subscription suspension, recovered on success
- Add RetryAnalytics: success rate, avg attempts to success, per-decline
  breakdown, 24-bucket UTC hourly success heatmap
- Extend Soroban charging contract (contracts/subscription/src/charging.rs)
  with on-chain retry queue, grace period tracking, and circuit-breaker state
- Add supporting types to contracts/types/src/lib.rs (RetryConfig,
  ChargeAttempt, ChargeStatus)
- Add comprehensive test suite (>80% coverage): 40+ cases covering policy
  management, all decline codes, circuit breaker, backoff, split suggestion,
  grace period lifecycle, reminder scheduling, analytics, and orchestrator
  integration

Closes #964

Author: Clement-coder