feat(neuron): add stage and prod k8s manifests + Argo CD apps

Completes the three-env setup (dev was added previously). Each env has
deployments, services, ingress, PVC, ConfigMap, and ExternalSecret pulling
from Vault. Prod has larger resource limits and 10Gi PVC. Vault secret at
secret/neuron-technologies/prod seeded separately.

servers/legion/k8s/neuron-technologies/prod/externalsecret.yaml

@@ -0,0 +1,18 @@
+apiVersion: external-secrets.io/v1beta1
+kind: ExternalSecret
+metadata:
+  name: neuron-prod-secrets
+  namespace: neuron-prod
+spec:
+  refreshInterval: 1h
+  secretStoreRef:
+    name: vault
+    kind: ClusterSecretStore
+  target:
+    name: neuron-prod-secrets
+    creationPolicy: Owner
+  data:
+    - secretKey: NEURON_WEBHOOK_SECRET
+      remoteRef:
+        key: secret/data/neuron-technologies/prod
+        property: gitea_webhook_secret