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]>

templates/accounts/login.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+{% block title %}Anmelden{% endblock %}
+{% block content %}
+<div class="narrow">
+  <h1>Anmelden</h1>
+  <form method="post" class="card" novalidate>
+    {% csrf_token %}
+    {% include "form_fields.html" %}
+    <input type="hidden" name="next" value="{{ next }}">
+    <button type="submit" class="button wide">Anmelden</button>
+  </form>
+  <p class="muted">Kein Konto? Die Registrierung ist nur mit einer Einladung möglich. Passwort vergessen? Bitte wende dich an den Admin.</p>
+</div>
+{% endblock %}