Extract payday calculation logic to shared utility module

[Copilot]

Addresses code duplication feedback on #484 where business logic for calculating days before/after payday was duplicated between PaymasterWorker and EmailTestServer.

Changes

• Created lib/utils/payday.ts: Extracted daysBeforePayday and daysAfterPayday functions as reusable utilities
• Updated workers/paymaster/src/index.ts: Removed duplicate methods, imports shared utilities
• Updated workers/email/scripts/emailOverview.ts: Replaced 39 lines of duplicated calculation logic with utility function call

Before/After

Before:

// emailOverview.ts - duplicated logic
let expectedPayDay: Date | null = null;
if (workspace.paidUntil) {
  expectedPayDay = new Date(workspace.paidUntil);
} else if (workspace.lastChargeDate) {
  const lastCharge = new Date(workspace.lastChargeDate);
  expectedPayDay = new Date(lastCharge.getFullYear(), lastCharge.getMonth() + 1, lastCharge.getDate());
}
// ... more calculation logic

After:

// emailOverview.ts - using shared utility
import { daysAfterPayday } from '../../../lib/utils/payday';

const days = daysAfterPayday(workspace.lastChargeDate, workspace.paidUntil);

Payday calculation logic is now maintained in a single location, ensuring consistency across all workers.

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