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/validate-env.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ENV_FILE="${1:?Usage: $0 <env-file>}"
+
+if [[ ! -f "$ENV_FILE" ]]; then
+  echo "Environment file not found: $ENV_FILE" >&2
+  exit 1
+fi
+
+set -a
+# shellcheck disable=SC1090
+source "$ENV_FILE"
+set +a
+
+DJANGO_ENV="${DJANGO_ENV:-prod}"
+
+required_vars=(
+  DJANGO_SECRET_KEY
+  DJANGO_ALLOWED_HOSTS
+  DB_PASSWORD
+  DB_NAME
+  DB_USER
+  DATABASE_URL
+)
+
+if [[ "$DJANGO_ENV" == "prod" || "$DJANGO_ENV" == "beta" ]]; then
+  required_vars+=(
+    RECAPTCHA_PUBLIC_KEY
+    RECAPTCHA_PRIVATE_KEY
+    EMAIL_HOST_USER
+    EMAIL_HOST_PASSWORD
+  )
+fi
+
+if [[ "$DJANGO_ENV" == "prod" ]]; then
+  required_vars+=(TIANJI_WEBSITE_ID)
+fi
+
+missing=()
+for var in "${required_vars[@]}"; do
+  if [[ -z "${!var:-}" ]]; then
+    missing+=("$var")
+  fi
+done
+
+if ((${#missing[@]} > 0)); then
+  echo "Missing required environment variables in $ENV_FILE:" >&2
+  printf '  - %s\n' "${missing[@]}" >&2
+  echo "Copy .env.prod.example to .env on the server and set production values." >&2
+  exit 1
+fi
+
+if [[ "$DJANGO_ENV" == "prod" && "$DJANGO_SECRET_KEY" == change-me* ]]; then
+  echo "DJANGO_SECRET_KEY must be changed from the example value for production." >&2
+  exit 1
+fi
+
+echo "Environment validation passed (DJANGO_ENV=$DJANGO_ENV)."