fix: resolve WebUI API proxy ECONNREFUSED and configmap placeholder issues

- Fix route handlers to use API_GATEWAY_URL env var with correct K8s fallback
  instead of localhost:3015
- Replace ${} placeholders in configmap.yaml with actual static values to
  prevent kustomize from overwriting envsubst-generated config
- Separate secrets into dedicated secrets.yaml for envsubst processing
- Update deploy.sh to only envsubst secrets (configmap now has static values)

Made-with: Cursor

backend/deploy/k8s/secrets.yaml

@@ -0,0 +1,16 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: gooseek-secrets
+  namespace: gooseek
+type: Opaque
+stringData:
+  OPENAI_API_KEY: "${OPENAI_API_KEY}"
+  ANTHROPIC_API_KEY: "${ANTHROPIC_API_KEY}"
+  GEMINI_API_KEY: "${GEMINI_API_KEY}"
+  JWT_SECRET: "${JWT_SECRET}"
+  TIMEWEB_API_KEY: "${TIMEWEB_API_KEY}"
+  OLLAMA_API_TOKEN: "${OLLAMA_API_TOKEN}"
+  SMTP_PASSWORD: "${SMTP_PASSWORD}"
+  POSTGRES_USER: "gooseek"
+  POSTGRES_PASSWORD: "gooseek"