Add device linking foundation

backend/prisma/migrations/20260624123000_milestone113_device_linking_foundation/migration.sql

@@ -0,0 +1,3 @@
+ALTER TABLE "devices"
+ADD COLUMN "linked_at" TIMESTAMP(3),
+ADD COLUMN "relinked_at" TIMESTAMP(3);

backend/prisma/schema.prisma

@@ -43,6 +43,8 @@ model Device {
   deviceName         String              @map("device_name")
   appVersion         String              @map("app_version")
   installTokenHash   String              @map("install_token_hash")
+  linkedAt           DateTime?           @map("linked_at")
+  relinkedAt         DateTime?           @map("relinked_at")
   tokenHash          String?             @unique @map("token_hash")
   tokenCreatedAt     DateTime?           @map("token_created_at")
   tokenLastUsedAt    DateTime?           @map("token_last_used_at")