feat: verify Turborepo pipeline — all 8 packages build and test

Adds packageManager field to root package.json (required by turbo v2.9),
loads .env via dotenv in API jest config so Prisma integration tests find
DATABASE_URL when run from the monorepo root. pnpm build + pnpm test: 8/8 passing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -13,6 +13,7 @@
     "@types/node": "^22.0.0",
     "prettier": "^3.0.0"
   },
+  "packageManager": "pnpm@10.6.2",
   "engines": {
     "node": ">=22.0.0",
     "pnpm": ">=10.0.0"