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]>

core/migrations/0001_initial.py

@@ -0,0 +1,23 @@
+# Generated by Django 5.2.18 on 2026-10-30 20:04
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='RateLimitBucket',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('key', models.CharField(max_length=255, unique=True)),
+                ('window_start', models.DateTimeField()),
+                ('count', models.PositiveIntegerField(default=0)),
+            ],
+        ),
+    ]