chore(neuron): retire Python backend — keep cloudflared only

Remove neuron/neuron-alpha/neuron-beta/neuron-gamma Deployments and
Services, the slot LB service, Python ingress, slot PVCs, snapshot job,
neuron-config ConfigMap, and neuron-secrets ExternalSecret. The Python
MCP backend has been superseded by the Spring Boot backend in neuron-prod.

Cloudflared Deployment and cloudflared-secret ExternalSecret are
retained in the neuron namespace. The neuron-data PVC is preserved
for data safety.

servers/legion/k8s/neuron/external-secrets.yaml

@@ -1,36 +1,3 @@
----
-# neuron-secrets — GITHUB_WEBHOOK_SECRET, GITEA_WEBHOOK_SECRET, SLACK_BOT_TOKEN, SLACK_SIGNING_SECRET
-apiVersion: external-secrets.io/v1beta1
-kind: ExternalSecret
-metadata:
-  name: neuron-secrets
-  namespace: neuron
-spec:
-  refreshInterval: 1h
-  secretStoreRef:
-    name: vault
-    kind: ClusterSecretStore
-  target:
-    name: neuron-secrets
-    creationPolicy: Owner
-  data:
-    - secretKey: GITHUB_WEBHOOK_SECRET
-      remoteRef:
-        key: secret/data/github
-        property: webhook_secret
-    - secretKey: GITEA_WEBHOOK_SECRET
-      remoteRef:
-        key: secret/data/gitea
-        property: api_token
-    - secretKey: SLACK_BOT_TOKEN
-      remoteRef:
-        key: secret/data/slack
-        property: bot_token
-    - secretKey: SLACK_SIGNING_SECRET
-      remoteRef:
-        key: secret/data/slack
-        property: signing_secret
----
 # cloudflared-secret — TUNNEL_TOKEN for the Cloudflare tunnel
 apiVersion: external-secrets.io/v1beta1
 kind: ExternalSecret