feat(db): add is_legacy column to referral table with migration

- Add is_legacy boolean column to referral table (default false)
- Migration adds referral_legacy enum value to grant_type
- Backfills existing referrals as is_legacy=true (grandfathered users)
- Migrates existing credit_ledger referral grants with expiry to referral_legacy type

Author: brandonkachen

packages/internal/src/db/migrations/0036_quiet_franklin_storm.sql

@@ -0,0 +1,16 @@
+ALTER TYPE "public"."grant_type" ADD VALUE 'referral_legacy' BEFORE 'purchase';--> statement-breakpoint
+ALTER TABLE "referral" ADD COLUMN "is_legacy" boolean DEFAULT false NOT NULL;--> statement-breakpoint
+-- Backfill: Mark all existing referrals as legacy (they were created under the old recurring program)
+UPDATE "referral" SET "is_legacy" = true;--> statement-breakpoint
+-- Migrate existing referral grants that have an expiry date to referral_legacy type
+-- (These are the recurring grants from the old program)
+UPDATE "credit_ledger" 
+SET "type" = 'referral_legacy', 
+    "priority" = 30
+WHERE "type" = 'referral' 
+  AND "expires_at" IS NOT NULL;--> statement-breakpoint
+-- Update priority for remaining referral grants (one-time grants, if any exist) to new priority
+UPDATE "credit_ledger"
+SET "priority" = 50
+WHERE "type" = 'referral'
+  AND "expires_at" IS NULL;