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

scripts/docker-entrypoint.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+cd /app/company_site
+
+wait_for_database() {
+  if [[ -z "${DATABASE_URL:-}" && -z "${DB_HOST:-}" ]]; then
+    return 0
+  fi
+
+  echo "Waiting for database..."
+  for _ in $(seq 1 30); do
+    if uv run python - <<'PY'
+import os
+import sys
+
+os.environ.setdefault("DJANGO_SETTINGS_MODULE", "company_site.settings")
+
+import django
+from django.db import connections
+from django.db.utils import OperationalError
+
+django.setup()
+
+try:
+    connections["default"].ensure_connection()
+except OperationalError:
+    sys.exit(1)
+PY
+    then
+      echo "Database is ready."
+      return 0
+    fi
+    sleep 2
+  done
+
+  echo "Database did not become ready in time." >&2
+  exit 1
+}
+
+wait_for_database
+
+uv run python manage.py migrate --noinput
+uv run python manage.py collectstatic --noinput
+
+exec uv run gunicorn company_site.wsgi:application \
+  --bind "${GUNICORN_BIND:-0.0.0.0:8000}" \
+  --workers "${GUNICORN_WORKERS:-2}"