feat(gotify): put the Gotify UI behind ZITADEL single sign-on

Gotify has no OIDC support, so it cannot validate a ZITADEL token itself. Put
oauth2-proxy in front of it and let nginx ask the proxy for every request.

The Gotify REST API keeps its own client-token check. /message, /application,
/client, /user, /version, /stream and /health are skipped by the proxy, so
Forgejo webhooks, the harness, poimen-sender and the Android client keep working
without a browser session. Only the web UI requires SSO.

The /oauth2 path needs its own ingress: the auth_request annotation would
otherwise also guard /oauth2/start, and the login entry point would demand a
login first.

The client id and secret come from project role provisioning:
`zitadel-provision.py apply` now creates the `gotify` OIDC app, and the secret is
SOPS-encrypted under k8s/argocd/secrets.

Gotify's own login remains behind the proxy. ZITADEL decides who reaches the
service, Gotify decides what that person may do inside it.

k8s/apps/gotify/oauth2-proxy-service.yaml

@@ -0,0 +1,15 @@
+apiVersion: v1
+kind: Service
+metadata:
+  name: gotify-oauth2-proxy
+  namespace: notifications
+  labels:
+    app: gotify-oauth2-proxy
+spec:
+  type: ClusterIP
+  selector:
+    app: gotify-oauth2-proxy
+  ports:
+    - name: http
+      port: 4180
+      targetPort: http