chore: isolate frontend and backend via npm workspaces and @mmapp/contracts

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

backend/package.json

@@ -5,7 +5,7 @@
   "type": "module",
   "scripts": {
     "dev": "tsx watch src/index.ts",
-    "build": "tsc",
+    "build": "npm run build -w @mmapp/contracts && tsc",
     "test:alert-engine": "tsx src/services/alert-engine/groups.test.ts && tsx src/services/alert-engine/evaluate-rule.test.ts && tsx src/services/alert-engine/confirm-stability.test.ts",
     "start": "node dist/index.js",
     "db:generate": "drizzle-kit generate",
@@ -13,6 +13,7 @@
     "db:studio": "drizzle-kit studio"
   },
   "dependencies": {
+    "@mmapp/contracts": "0.1.0",
     "@fastify/cors": "^11.2.0",
     "@fastify/type-provider-zod": "^1.0.0",
     "better-sqlite3": "^12.9.0",