Dockerize Django app with dev/beta/prod env config and uv (#6)

## Summary

- Containerize the Django app with Docker and docker-compose (dev + production)
- Refactor settings into `dev` / `beta` / `prod` environments driven by environment variables
- Connect to PostgreSQL via `DATABASE_URL` or `DB_*` vars
- Migrate package management from pip to uv (`pyproject.toml`, `uv.lock`)
- Split Gitea workflows: PRs run unit tests only; pushes to `master` run tests, Docker validation, and deploy
- Update deploy script to rsync code, preserve server `.env`, validate config, and run Docker compose

Closes #4

## Test plan

- [x] `uv run python manage.py test` passes locally (10/10)
- [x] `DJANGO_ENV=beta` and `DJANGO_ENV=prod` load with correct logging levels
- [x] `scripts/validate-env.sh` rejects missing production variables
- [ ] `docker compose up --build` starts app + Postgres locally
- [ ] Containerized unit tests pass in CI Docker job
- [ ] Server `.env` created from `.env.prod.example` before first production deploy
- [ ] CI workflow runs on this PR (tests only, no deploy)

Reviewed-on: #6

Dockerfile

@@ -0,0 +1,28 @@
+FROM python:3.12-slim
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    UV_COMPILE_BYTECODE=1 \
+    UV_LINK_MODE=copy \
+    UV_PROJECT_ENVIRONMENT=/app/.venv
+
+WORKDIR /app
+
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends libpq5 \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY --from=ghcr.io/astral-sh/uv:latest /uv /usr/local/bin/uv
+
+COPY pyproject.toml uv.lock ./
+RUN uv sync --frozen --no-dev
+
+COPY company_site ./company_site
+COPY scripts/docker-entrypoint.sh /entrypoint.sh
+RUN chmod +x /entrypoint.sh
+
+WORKDIR /app/company_site
+
+EXPOSE 8000
+
+ENTRYPOINT ["/entrypoint.sh"]