docs: update scaling, health probe, and beat scheduler documentation

- Update DeploymentGuide.md with scaling instructions and beat service info
- Update KubernetesDeployment.md with unauthenticated probe paths and beat note
- Update ProductionReadiness.md with new health endpoints table and beat guidance
- Update API.md with new healthz/live and healthz/ready endpoint docs

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

docs/DeploymentGuide.md

@@ -349,16 +349,18 @@ workdir:
 
 ## Scaling
 
+DocuElevate is designed for horizontal scaling.  Both API and worker pods are stateless and can be scaled independently.
+
 ### Docker Compose
 
-Add more worker containers:
+Scale workers (task processing) and API pods (request handling) independently:
 
-```yaml
-worker:
-  deploy:
-    replicas: 3
+```bash
+docker compose up -d --scale worker=3 --scale api=2
 ```
 
+> **Note:** The `beat` service (Celery Beat scheduler) must always run as exactly **one** instance.  Do not scale it.  It publishes periodic tasks to the Redis broker; workers pick them up.
+
 ### Kubernetes / Helm
 
 Enable HPA:
@@ -377,13 +379,15 @@ worker:
     maxReplicas: 10
 ```
 
+The Helm chart deploys a separate **beat** pod (always 1 replica, `Recreate` strategy) so that scheduled tasks are never duplicated when workers scale.
+
 ---
 
 ## Monitoring
 
 - **Docker Compose**: `docker-compose logs -f`, `docker stats`
 - **Kubernetes**: `kubectl logs -l app.kubernetes.io/component=api -f`
-- **Prometheus / Grafana**: Scrape the `/api/health` endpoint for readiness; add custom metrics as needed.
+- **Prometheus / Grafana**: Scrape the `/api/diagnostic/healthz/ready` endpoint for readiness; add custom metrics as needed.
 - **Uptime Kuma**: Set `UPTIME_KUMA_URL` to your push URL for heartbeat monitoring.
 
 ---