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

.env.example

@@ -0,0 +1,37 @@
+# Local development environment file (copy to .env).
+# For production server values, use .env.prod.example instead.
+DJANGO_ENV=dev
+DJANGO_DEBUG=true
+DJANGO_SECRET_KEY=change-me-for-local-development
+DJANGO_ALLOWED_HOSTS=localhost,127.0.0.1,0.0.0.0
+
+# Database (docker-compose sets DATABASE_URL for the web service)
+DATABASE_URL=postgres://company_site:company_site@db:5432/company_site
+# DB_HOST=db
+# DB_PORT=5432
+# DB_NAME=company_site
+# DB_USER=company_site
+# DB_PASSWORD=company_site
+
+# Logging
+# DJANGO_LOG_LEVEL=DEBUG
+
+# Analytics
+TIANJI_ENABLED=false
+TIANJI_TRACKER_URL=https://tianji.aimloperations.com/tracker.js
+TIANJI_WEBSITE_ID=
+
+# reCAPTCHA
+RECAPTCHA_PUBLIC_KEY=
+RECAPTCHA_PRIVATE_KEY=
+
+# Email (SMTP2GO)
+EMAIL_HOST=mail.smtp2go.com
+EMAIL_HOST_USER=
+EMAIL_HOST_PASSWORD=
+EMAIL_PORT=2525
+EMAIL_USE_TLS=true
+
+# Gunicorn
+GUNICORN_WORKERS=2
+GUNICORN_BIND=0.0.0.0:8000