refactor: move vpn-dns config directory from /etc/coredns to /tmp/nexavpn-vpn-dns

Add configDir, corefilePath, and overridesPath constants pointing to /tmp/nexavpn-vpn-dns directory. Update all file path references in writeCorefile and refreshOverrides to use new constants instead of hardcoded /etc/coredns paths.

deploy/vpn-dns/main.go

@@ -14,6 +14,12 @@ import (
 	"time"
 )
 
+const (
+	configDir      = "/tmp/nexavpn-vpn-dns"
+	corefilePath   = configDir + "/Corefile"
+	overridesPath  = configDir + "/service-overrides.hosts"
+)
+
 type dnsResponse struct {
 	Records []dnsRecord `json:"records"`
 }
@@ -26,7 +32,7 @@ type dnsRecord struct {
 func main() {
 	ctx := context.Background()
 
-	if err := os.MkdirAll("/etc/coredns", 0o755); err != nil {
+	if err := os.MkdirAll(configDir, 0o755); err != nil {
 		log.Fatalf("unable to create coredns config dir: %v", err)
 	}
 
@@ -47,7 +53,7 @@ func main() {
 		}
 	}()
 
-	cmd := exec.Command("/coredns", "-conf", "/etc/coredns/Corefile")
+	cmd := exec.Command("/coredns", "-conf", corefilePath)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
 
@@ -65,7 +71,7 @@ func writeCorefile() error {
 
 	corefile := fmt.Sprintf(`%s {
     errors
-    hosts /etc/coredns/service-overrides.hosts {
+    hosts ` + overridesPath + ` {
         ttl 30
         reload 15s
         fallthrough
@@ -75,7 +81,7 @@ func writeCorefile() error {
 }
 `, envOrDefault("NEXAVPN_VPN_DNS_ADDR", ":53"), strings.Join(upstreams, " "))
 
-	return os.WriteFile("/etc/coredns/Corefile", []byte(corefile), 0o644)
+	return os.WriteFile(corefilePath, []byte(corefile), 0o644)
 }
 
 func refreshOverrides(ctx context.Context) error {
@@ -116,7 +122,7 @@ func refreshOverrides(ctx context.Context) error {
 		content += "\n"
 	}
 
-	return os.WriteFile(filepath.Clean("/etc/coredns/service-overrides.hosts"), []byte(content), 0o644)
+	return os.WriteFile(filepath.Clean(overridesPath), []byte(content), 0o644)
 }
 
 func parseList(raw string) []string {