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/migrations/20260303_payment_scheduling/migration.sql

@@ -0,0 +1,10 @@
+-- Payment scheduling: pledges can have a future due date and installment plans
+ALTER TABLE "Pledge" ADD COLUMN "dueDate" TIMESTAMP(3);
+ALTER TABLE "Pledge" ADD COLUMN "planId" TEXT;
+ALTER TABLE "Pledge" ADD COLUMN "installmentNumber" INTEGER;
+ALTER TABLE "Pledge" ADD COLUMN "installmentTotal" INTEGER;
+ALTER TABLE "Pledge" ADD COLUMN "reminderSentForDueDate" BOOLEAN NOT NULL DEFAULT false;
+
+-- Index for finding pledges due today/upcoming
+CREATE INDEX "Pledge_dueDate_status_idx" ON "Pledge"("dueDate", "status");
+CREATE INDEX "Pledge_planId_idx" ON "Pledge"("planId");