docs: update README + add docs

[kowyo-bot]

• Update README to match current implementation (Next.js + Better Auth + Postgres chat history + Vercel deploy)
• Add simple technical docs under docs/ (architecture/database/deployment)

[gemini-code-assist]

Summary of Changes

Hello @kowyo-bot, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

此拉取请求旨在全面更新项目的文档，使其与当前基于 Next.js、Better Auth、PostgreSQL 聊天历史持久化以及 Vercel 部署的实现保持一致。它引入了详细的技术文档，涵盖了系统架构、数据库结构和部署指南，从而为开发者和未来的贡献者提供了清晰的指引。

Highlights

• 文档更新: 更新了 README.md 文件，以反映项目当前的技术栈和本地开发指南。
• 新增技术文档: 新增了 docs/ 目录下的技术文档，详细介绍了项目架构、数据库设计和部署流程。

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[gemini-code-assist]

Code Review

The pull request significantly updates the project's documentation. The README.md has been revamped to reflect the current implementation, including details on the tech stack, local development setup, and a new section pointing to comprehensive technical documentation. New documentation files (architecture.md, database.md, deployment.md) have been added under the docs/ directory, providing detailed insights into the project's architecture, database schema and migrations, and deployment procedures for Vercel and GitHub Actions. These changes greatly improve the project's clarity and ease of setup for new contributors or users. The documentation is clear, accurate, and well-structured, and no issues of medium, high, or critical severity were found.