Document OAuth2 admin protection

docs/OAUTH2-AUTH.md

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+# OAuth2/OIDC auth for admin.familyfed.ie
+
+`admin.familyfed.ie` must be protected before traffic reaches the static Astro
+files. Use Pocket ID as the OIDC issuer, with an OAuth2/OIDC proxy in front of
+the generated admin page.
+
+Required policy:
+
+- Host: `admin.familyfed.ie`
+- Required Pocket ID group: `familyfed_admin`
+- Static upstream/root: `dist/admin/index.html`
+- OIDC issuer: `https://auth.bcgen.ie`
+
+## Pocket ID client
+
+Create a Pocket ID OIDC client:
+
+- Name: `FamilyFed Admin`
+- Redirect URL: `https://admin.familyfed.ie/oauth2/callback`
+- Scopes: `openid email profile groups`
+- Allowed user group: `familyfed_admin`
+
+The redirect URL must exactly match the OAuth2 proxy config. If the proxy is
+mounted on a different hostname or path, use that callback URL in both places.
+
+## OAuth2 proxy setup
+
+Run an OAuth2/OIDC proxy in front of the static admin output. The proxy should
+receive all traffic for `admin.familyfed.ie`, complete the Pocket ID login, check
+the `groups` claim, and only then forward to the static admin page.
+
+Example `oauth2-proxy` arguments:
+
+```text
+--http-address=0.0.0.0:4180
+--provider=oidc
+--oidc-issuer-url=https://auth.bcgen.ie
+--redirect-url=https://admin.familyfed.ie/oauth2/callback
+--scope=openid email profile groups
+--email-domain=*
+--allowed-group=familyfed_admin
+--oidc-groups-claim=groups
+--reverse-proxy=true
+--upstream=http://127.0.0.1:8080/
+--cookie-secure=true
+--cookie-samesite=lax
+--cookie-name=__Secure-familyfed_admin_oauth2
+--skip-provider-button=true
+--set-xauthrequest=true
+--pass-user-headers=true
+```
+
+Required secret environment values:
+
+```text
+OAUTH2_PROXY_CLIENT_ID=<pocket-id-client-id>
+OAUTH2_PROXY_CLIENT_SECRET=<pocket-id-client-secret>
+OAUTH2_PROXY_COOKIE_SECRET=<generated-cookie-secret>
+```
+
+Generate a cookie secret with:
+
+```bash
+openssl rand -base64 32
+```
+
+The upstream can be any local static file server or reverse-proxy target that
+serves `dist/admin/index.html` as the root document for the admin hostname. Do
+not expose that upstream directly to the public internet.
+
+## Deployment checks
+
+The site build verifies that `dist/admin/index.html` exists. That confirms the
+static admin page is available for the host, but it does not prove the external
+proxy has enabled Pocket ID.
+
+Verify live protection with:
+
+```bash
+curl -I https://admin.familyfed.ie/
+```
+
+Expected unauthenticated behavior is a redirect into the OAuth2/OIDC login flow
+or a `401`/`403` response from the auth proxy. Users outside
+`familyfed_admin` should never reach the static admin HTML.