feat(experiment-platform): wire backend sealed secret to deployment

- Add envFrom: secretRef for experiment-platform-backend-secret
- Register backend-sealed-secret.yaml in prod kustomization

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: soohyunme

services/experiment-platform-pseudolab/base/backend-deployment.yaml

@@ -22,3 +22,6 @@ spec:
           imagePullPolicy: Always
           ports:
             - containerPort: 8000
+          envFrom:
+            - secretRef:
+                name: experiment-platform-backend-secret

services/experiment-platform-pseudolab/overlays/prod/kustomization.yaml

@@ -6,6 +6,7 @@ resources:
   - namespace.yaml
   - ingress.yaml
   - ghcr-pull-secret.yaml
+  - backend-sealed-secret.yaml
 commonLabels:
   environment: production
   project: experiment-platform