docs: add concise ariadne README

README.md

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+# ariadne
+
+Ariadne is the Atlas admin and account automation service.
+
+It sits behind the portal and handles the jobs that are annoying or risky to do
+by hand: approving access, syncing account state, rotating service passwords,
+cleaning stale Kubernetes work, checking platform health, and keeping a few
+service integrations lined up.
+
+## How it works
+
+Ariadne is a FastAPI service with a small scheduler. It talks to Keycloak,
+Vault, Mailu, Nextcloud, Wger, Firefly, Jenkins, Metis, Kubernetes, and a few
+Atlas-specific services through focused adapters under `ariadne/services/`.
+
+The API is split between admin routes, account self-service routes, internal
+event hooks, and Prometheus metrics. Background jobs store run history in the
+Ariadne database so failures can be inspected later instead of vanishing into
+logs.
+
+Useful routes:
+
+- `GET /health`
+- `GET /metrics`
+- `GET /api/admin/cluster/state`
+- `POST /api/admin/access/requests/{username}/approve`
+- `POST /api/account/mailu/rotate`
+- `POST /api/account/wger/reset`
+- `POST /api/account/firefly/reset`
+- `POST /events`
+
+## Development
+
+```bash
+python -m pytest
+ruff check .
+```
+
+Most runtime behavior is configured through environment variables in
+`ariadne/settings.py`. Keep service-specific logic in the small adapter modules;
+`ariadne/app.py` should stay focused on request flow and task orchestration.