fix: stage save errors and add milestone progress field

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/prisma/migrations/20260605130000_milestone_progress/migration.sql

@@ -0,0 +1 @@
+ALTER TABLE "milestones" ADD COLUMN IF NOT EXISTS "progress" INTEGER NOT NULL DEFAULT 0;

backend/prisma/schema.prisma

@@ -162,6 +162,7 @@ model Milestone {
   description String?
   startDate   DateTime?
   dueDate     DateTime?
+  progress    Int       @default(0)
   links       String?   // JSON: [{ "label": string, "url": string }]
   completedAt DateTime?
   order       Int       @default(0)