feat: update K8s manifests to use regcred secret

Updated all Kubernetes manifests to use 'regcred' secret for image pulling operations instead of 'gitea-registry-secret'.

Changes:
- backend-deployment.yaml: Updated imagePullSecrets to use regcred
- frontend-deployment.yaml: Updated imagePullSecrets to use regcred
- image-pull-secret.yaml: Updated secret name to regcred
- DEPLOYMENT_GUIDE.md: Updated documentation references

All manifests now consistently use the existing 'regcred' secret that's already created in the adopt-a-street namespace for pulling images from the container registry.

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Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

deploy/k8s/image-pull-secret.yaml

@@ -1,7 +1,7 @@
 apiVersion: v1
 kind: Secret
 metadata:
-  name: gitea-registry-secret
+  name: regcred
   namespace: adopt-a-street
 type: kubernetes.io/dockerconfigjson
 data:
@@ -13,7 +13,7 @@ data:
 # 2. Update the base64 encoded .dockerconfigjson with your credentials
 # 3. Apply with: kubectl apply -f image-pull-secret.yaml
 # 4. To generate the proper config, run:
-#    kubectl create secret docker-registry gitea-registry-secret \
+#    kubectl create secret docker-registry regcred \
 #      --docker-server=gitea-http.taildb3494.ts.net \
 #      --docker-username=will \
 #      --docker-password=YOUR_GITEA_PASSWORD \