Add phase 1: Docker setup, auth with invitations and roles

- Django 5.2 + PostgreSQL 17 via docker compose (app + db, volumes,
  healthcheck on /healthz, test profile running pytest against Postgres)
- Custom e-mail user model with user/admin roles
- Invitation-only registration with single-use, hashed, expiring links
- Admin area: invitations, role changes, deactivation, password reset
  via one-time link or temporary password (forced change on login)
- First admin created from ADMIN_EMAIL/ADMIN_PASSWORD on startup
- Login rate limiting per e-mail and IP (DB-backed, worker-safe)
- Proxy middleware honouring X-Forwarded-* only from TRUSTED_PROXIES,
  optional CLIENT_IP_HEADER (e.g. CF-Connecting-IP)
- Security headers (CSP, HSTS, Permissions-Policy), secure __Host- cookies
- Mobile-first German UI, CLAUDE.md, README, init-env script
- 65 tests for auth, permissions, proxy handling and rate limiting

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Dockerfile

@@ -0,0 +1,35 @@
+FROM python:3.12-slim AS base
+
+ENV PYTHONDONTWRITEBYTECODE=1 \
+    PYTHONUNBUFFERED=1 \
+    PIP_NO_CACHE_DIR=1 \
+    PIP_DISABLE_PIP_VERSION_CHECK=1
+
+RUN groupadd --system --gid 1000 app \
+    && useradd --system --uid 1000 --gid app --home /app app
+
+WORKDIR /app
+COPY requirements.txt .
+RUN pip install -r requirements.txt
+
+COPY . .
+RUN sed -i 's/\r$//' docker/entrypoint.sh && chmod +x docker/entrypoint.sh \
+    && SECRET_KEY=build-only python manage.py collectstatic --noinput \
+    && mkdir -p /data/media && chown -R app:app /data
+
+USER app
+EXPOSE 8000
+HEALTHCHECK --interval=30s --timeout=5s --start-period=30s --retries=3 \
+    CMD python -c "import urllib.request,sys; sys.exit(0 if urllib.request.urlopen('http://127.0.0.1:8000/healthz', timeout=4).status == 200 else 1)"
+ENTRYPOINT ["/app/docker/entrypoint.sh"]
+
+# Test image: same app plus dev dependencies (docker compose --profile test run --rm test)
+FROM base AS test
+USER root
+COPY requirements-dev.txt .
+RUN pip install -r requirements-dev.txt
+USER app
+ENTRYPOINT []
+CMD ["pytest"]
+
+FROM base AS production