feat: allow sitters to mark jobs as completed with notification

prisma/migrations/20260112221434_add_booking_completed_at/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "Booking" ADD COLUMN "completedAt" DATETIME;

prisma/schema.prisma

@@ -30,6 +30,7 @@ model Booking {
   date       DateTime
   sitterName String?
   type       String   @default("SITTER") // "SITTER" or "OWNER_HOME"
+  completedAt DateTime?
   createdAt  DateTime @default(now())
   
   @@unique([planId, date, sitterName])