Fix authentik worker OOMKill: limit concurrency to 2

Dramatiq defaults to one worker process per CPU core. On ringtail (16 cores)
this spawned 16 processes, each loading the full Django app, exceeding the
1Gi memory limit and causing a crash loop (228 restarts over 7 days).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

argocd/manifests/authentik/deployment-worker.yaml

@@ -53,6 +53,8 @@ spec:
                   key: postgresql-password
             - name: AUTHENTIK_REDIS__HOST
               value: authentik-redis
+            - name: AUTHENTIK_WORKER_CONCURRENCY
+              value: "2"
             - name: AUTHENTIK_GRAFANA_CLIENT_SECRET
               valueFrom:
                 secretKeyRef: