feat(auth): add Authentik OIDC adapter

Refs #96

docs/plans/authentik-sso-setup.md

@@ -0,0 +1,40 @@
+# Authentik SSO Setup
+
+## Create the Authentik application
+
+1. In Authentik, create an OAuth2/OpenID Provider.
+2. Create an Application and link it to that provider.
+3. Copy the generated client ID and client secret.
+
+## Required environment variables
+
+Set these values for the gateway/auth runtime:
+
+```bash
+AUTHENTIK_CLIENT_ID=your-client-id
+AUTHENTIK_CLIENT_SECRET=your-client-secret
+AUTHENTIK_ISSUER=https://authentik.example.com
+```
+
+`AUTHENTIK_ISSUER` should be the Authentik base URL, for example `https://authentik.example.com`.
+
+## Redirect URI
+
+Configure this redirect URI in the Authentik provider/application:
+
+```text
+{BETTER_AUTH_URL}/api/auth/callback/authentik
+```
+
+Example:
+
+```text
+https://mosaic.example.com/api/auth/callback/authentik
+```
+
+## Test the flow
+
+1. Start the gateway with `BETTER_AUTH_URL` and the Authentik environment variables set.
+2. Open the Mosaic login flow and choose the Authentik provider.
+3. Complete the Authentik login.
+4. Confirm the browser returns to Mosaic and a session is created successfully.