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>

Dockerfile.backend

@@ -14,6 +14,9 @@ COPY packages/backend/package.json ./packages/backend/package.json
 
 RUN pnpm install --frozen-lockfile --filter @sexy.pivoine.art/backend
 
+# Rebuild argon2 native bindings (pnpm v10 build approval bypassed explicitly)
+RUN pnpm rebuild argon2
+
 COPY packages/backend ./packages/backend
 
 RUN pnpm --filter @sexy.pivoine.art/backend build