提取共享 API 和类型层

[yeyixiang2007]

What to build

Extract and stabilize the shared API contract and frontend type definitions so web, native app, and mini program clients can consume the same backend data model without duplicating schema knowledge.
This slice should make the data contract reusable across clients while keeping the current web app working.

Acceptance criteria

• Shared request/response types are available from a single package or module.
• The web client continues to build and run against the shared contract.
• The shared layer is usable by future Android/iOS and mini program clients.

Blocked by

• 确定多端技术路线 #233