Cache expiration_time in SubscriptionLedger to eliminate redundant arithmetic on every call

[Copilot]

Addresses performance feedback from #381 (#381 (comment)). For Daily subscriptions, subscription.issue_time + duration_ms was computed on every call() invocation.

Changes

• Added expiration_time: Option<Moment> field to SubscriptionLedger

  • Some(issue_time + days * DAYS_TO_MS) for Daily subscriptions
  • None for Lifetime subscriptions

• Updated call() to use cached value instead of recalculating expiration on each invocation

• Extended v1→v2 migration to compute and populate expiration_time for existing Daily subscriptions

// Before: computed on every call
SubscriptionMode::Daily { days } => {
    let duration_ms = Moment::from(days * DAYS_TO_MS);
    if now < subscription.issue_time + duration_ms { ... }
}

// After: cached at subscription creation
SubscriptionMode::Daily { .. } => {
    if let Some(ref expiration_time) = subscription.expiration_time {
        if now < *expiration_time { ... }
    }
}

All 27 tests pass.

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