feat: deferred payments & installment plans — pledge = promise to pay on a date

CORE PRODUCT SHIFT:
A pledge is now a promise to pay on a future date, not just 'pay now'.

NEW FLOW: Amount → Schedule → Payment/Identity → Confirmation

SCHEDULE STEP (/p/[token] step 1):
- 'Pay right now' — existing card/DD/bank flow
- 'Pay on a specific date' — calendar picker with smart suggestions
  (This Friday, End of month, Payday 1st, In 2 weeks, In 1 month)
- 'Split into monthly payments' — 2/3/4/6/12 month installment plans
  with per-installment breakdown and date schedule

SCHEMA CHANGES:
- Pledge.dueDate — when the donor promises to pay (null = now)
- Pledge.planId — groups installment pledges together
- Pledge.installmentNumber / installmentTotal — e.g. 2 of 4
- Pledge.reminderSentForDueDate — tracking flag
- New indexes on dueDate+status and planId

INSTALLMENT PLANS:
- Creates N linked Pledge records with shared planId
- Each installment gets its own reference, due date, reminders
- Reminders: 2 days before, on due date, 3 days after, 10 days after
- WhatsApp receipt shows full plan summary

DEFERRED SINGLE PLEDGES:
- Reminders anchored to due date, not creation date
- 'Pay on date' → reminders: 2 days before, on day, +3d nudge, +10d final
- WhatsApp preferred when phone number provided

DASHBOARD:
- API returns dueDate, planId, installment info for each pledge
- Confirmation step shows schedule details for deferred pledges

pledge-now-pay-later/prisma/schema.prisma

@@ -100,6 +100,13 @@ model Pledge {
   iPaidClickedAt DateTime?
   notes          String?
 
+  // Payment scheduling — the core of "pledge now, pay later"
+  dueDate              DateTime? // null = pay now, set = promise to pay on this date
+  planId               String?   // groups installments together
+  installmentNumber    Int?      // e.g. 1 (of 4)
+  installmentTotal     Int?      // e.g. 4
+  reminderSentForDueDate Boolean @default(false)
+
   eventId        String
   event          Event    @relation(fields: [eventId], references: [id])
   qrSourceId     String?
@@ -121,6 +128,8 @@ model Pledge {
   @@index([eventId, status])
   @@index([donorEmail])
   @@index([donorPhone])
+  @@index([dueDate, status])
+  @@index([planId])
 }
 
 model PaymentInstruction {