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

.gitea/workflows/unittests.yml

@@ -13,13 +13,18 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python environment
+      - name: Install uv
         run: |
-          python3 -m venv .venv
-          .venv/bin/pip install --upgrade pip
-          .venv/bin/pip install -r requirements.txt
+          curl -LsSf https://astral.sh/uv/install.sh | sh
+          echo "$HOME/.local/bin" >> "$GITHUB_PATH"
+
+      - name: Install dependencies
+        run: uv sync --frozen
 
       - name: Run unit tests
+        env:
+          DJANGO_ENV: dev
+          DJANGO_SECRET_KEY: test-secret-key
         run: |
           cd company_site
-          ../.venv/bin/python manage.py test
+          uv run python manage.py test