[codex] add student miniapp MVP and mobile API

[2002yy]

Summary

• Add a FastAPI /api/mobile adapter for student login, task lookup, homework upload, evaluation, history, and progress.
• Add a uni-app + Vue 3 + Pinia student miniapp MVP under miniapp-student.
• Add an integration test covering the mobile student flow from login through evaluation result retrieval.

Why

The project is moving toward a split experience: teacher workflows stay on the existing web app while student workflows move to a WeChat mini program. This commit establishes the first runnable MVP skeleton without rewriting the existing backend or teacher web app.

Validation

• python -m pytest
• python -m pytest tests/test_mobile_api.py
• npm.cmd run build:mp-weixin in miniapp-student

Notes

The generated dist output and node_modules directory were not committed. To inspect the miniapp in WeChat Developer Tools, run npm.cmd install --no-audit --no-fund and npm.cmd run build:mp-weixin in miniapp-student, then import miniapp-student/dist/build/mp-weixin.