feat: Extract Wild Central as standalone Go service

Extract the Central networking functionality from wild-cloud/api into
a standalone service. Wild Central manages DNS (dnsmasq), gateway
(HAProxy), firewall (nftables), VPN (WireGuard), TLS (certbot),
security (CrowdSec), and DDNS — all the network appliance concerns.

All Cloud-specific code (instances, clusters, nodes, apps, backups,
operations, kubectl/talosctl tooling) has been removed. The API struct
and route registration contain only Central endpoints. Tests updated
to match the new API signature.

Builds and all tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/api/v1/response.go

@@ -0,0 +1,36 @@
+package v1
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+// APIResponse is the standard response envelope for all API endpoints.
+// All successful responses wrap data in the Data field.
+// Error responses use the Error field.
+// Message-only responses use the Message field.
+type APIResponse struct {
+	Data    any `json:"data,omitempty"`
+	Message string      `json:"message,omitempty"`
+	Error   string      `json:"error,omitempty"`
+}
+
+// respondJSON writes a JSON response with the given status code.
+// This is the low-level helper that all other response helpers use.
+func respondJSON(w http.ResponseWriter, status int, data any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(data)
+}
+
+// respondMessage writes a success response with only a message (no data).
+// Use this for operations that succeed but don't return meaningful data.
+func respondMessage(w http.ResponseWriter, status int, message string) {
+	respondJSON(w, status, APIResponse{Message: message})
+}
+
+// respondError writes an error response with the given status code and message.
+func respondError(w http.ResponseWriter, status int, message string) {
+	respondJSON(w, status, APIResponse{Error: message})
+}
+