fix: unblock Render deploy when Prisma migrations fail

Add missing milestones migration and fall back to db push so backend updates deploy successfully.

backend/prisma/migrations/20260605115000_create_milestones/migration.sql

@@ -0,0 +1,26 @@
+-- CreateTable (누락됐던 milestones 테이블 생성)
+CREATE TABLE IF NOT EXISTS "milestones" (
+    "id" TEXT NOT NULL,
+    "taskId" TEXT NOT NULL,
+    "title" TEXT NOT NULL,
+    "description" TEXT,
+    "startDate" TIMESTAMP(3),
+    "dueDate" TIMESTAMP(3),
+    "progress" INTEGER NOT NULL DEFAULT 0,
+    "links" TEXT,
+    "completedAt" TIMESTAMP(3),
+    "order" INTEGER NOT NULL DEFAULT 0,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "milestones_pkey" PRIMARY KEY ("id")
+);
+
+CREATE INDEX IF NOT EXISTS "milestones_taskId_idx" ON "milestones"("taskId");
+
+DO $$ BEGIN
+  ALTER TABLE "milestones" ADD CONSTRAINT "milestones_taskId_fkey"
+    FOREIGN KEY ("taskId") REFERENCES "tasks"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+EXCEPTION
+  WHEN duplicate_object THEN NULL;
+END $$;

render.yaml

@@ -3,7 +3,7 @@ services:
     name: eene-dashboard-backend
     runtime: node
     rootDir: backend
-    buildCommand: npm install --include=dev && npx prisma migrate deploy && npx prisma generate && npm run build
+    buildCommand: npm install --include=dev && npx prisma migrate deploy || npx prisma db push && npx prisma generate && npm run build
     startCommand: npm start
     envVars:
       - key: DATABASE_URL