fix: resolve pnpm frozen-lockfile error and argon2 native build

- Run pnpm install to update lockfile with packages/backend dependencies
- Add argon2 to root onlyBuiltDependencies (pnpm-workspace.yaml + package.json)
- Add explicit `pnpm rebuild argon2` in Dockerfile.backend to ensure native
  bindings compile regardless of pnpm v10 build approval state
- Remove pnpm.onlyBuiltDependencies from packages/backend/package.json
  (ineffective in workspace packages, warned by pnpm)

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

packages/backend/package.json

@@ -34,11 +34,6 @@
     "slugify": "^1.6.6",
     "uuid": "^11.1.0"
   },
-  "pnpm": {
-    "onlyBuiltDependencies": [
-      "argon2"
-    ]
-  },
   "devDependencies": {
     "@types/fluent-ffmpeg": "^2.1.27",
     "@types/nodemailer": "^6.4.17",