chore: migrate backend to uv package manager

[black-pwq]

Summary

This PR migrates the backend from plain pip + requirements.txt to uv, a fast Python package manager. All 211 backend tests pass.

Changes

src/backend/pyproject.toml (new)

Adds a standard pyproject.toml declaring the project metadata and pinned dependencies. Replaces requirements.txt as the source of truth for the backend's dependency set. A [dependency-groups] dev section lists test-only packages (pytest, httpx).

src/backend/uv.lock (new)

Lock file generated by uv, ensuring fully reproducible installs across all environments.

src/backend/Dockerfile

• Install uv via pip in the base image layer.
• Replace COPY requirements.txt + pip install -r requirements.txt with COPY pyproject.toml + uv sync --no-dev.
• Change the CMD from bare uvicorn to uv run uvicorn so the server runs inside the uv-managed virtual environment.

src/docker-compose.yml

Update the backend service command override from uvicorn ... to uv run uvicorn ... to match the Dockerfile change.

src/backend/main.py

Add load_dotenv() at startup so environment variables defined in a .env file are loaded before config.settings is evaluated. This fixes a subtle ordering issue when running locally outside of Docker.

src/backend/services/git_service.py + tests/test_git_service.py

Introduce a module-level alias _sleep = time.sleep and patch services.git_service._sleep in tests instead of services.git_service.time.sleep. The old patch target replaced time.sleep globally, causing anyio's internal thread-pool scheduler (which also calls time.sleep) to inflate the mock call count from 1 to 32, breaking the assertion. Patching the private alias isolates the mock to git_service only.

README.md / README.zh-CN.md

Update the Local Development backend setup instructions to reflect the uv workflow:

• Remove manual venv creation (python3.12 -m venv .venv && source .venv/bin/activate) and pip install -r requirements-dev.txt.
• Replace uvicorn main:app --reload … with uv run uvicorn main:app --reload ….
• Add a note explaining that uv auto-creates the venv from pyproject.toml on first run and that uv sync installs dev dependencies explicitly.

[black-pwq]

073e552 之后新增了以下改动：

src/backend/pyproject.toml / uv.lock

将依赖 dotenv 更正为正确的包名 python-dotenv，并更新 lock 文件。

src/backend/Dockerfile

改用 COPY --from=ghcr.io/astral-sh/uv:latest /uv /bin/uv 安装 uv，替代原来的 pip install uv，避免在生产镜像中保留 pip 层。

.github/workflows/ci.yml

将后端 CI 依赖安装从 pip install -r requirements-dev.txt 迁移至 astral-sh/setup-uv action + uv sync --frozen；测试命令改为 uv run python -m pytest。

.github/dependabot.yml

将后端包生态从 pip 改为 uv，避免 Dependabot 因找不到 requirements.txt 而报错；同步更新注释中的文件名引用。

src/backend/requirements.txt / requirements-dev.txt (已删除)

删除迁移后不再使用的两个文件，消除与 pyproject.toml 的歧义。

docs/project-structure.md

将后端目录树中的 requirements.txt 条目替换为 pyproject.toml 和 uv.lock 的描述。

README.md / README.zh-CN.md

• 本地开发：在后端步骤前新增 uv 安装说明（附官方文档链接和 curl 安装命令）。
• Testing 节：将 python -m pytest tests/ -v 改为 uv run pytest tests/ -v，避免在未激活 venv 时报错。