refactor: modernize Go idioms across codebase

Replace map[string]interface{} with map[string]any, ioutil.ReadAll with
io.ReadAll, and simplify redundant fmt.Sprintf/w.Write calls.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

addy.go

@@ -12,6 +12,7 @@ import (
 	"log"
 	"net/http"
 	"os"
+	"slices"
 	"strings"
 )
 
@@ -134,14 +135,7 @@ func addyAliasAPI(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "Alias creation is not configured", http.StatusServiceUnavailable)
 		return
 	}
-	domainAllowed := false
-	for _, d := range allowedAliasDomains {
-		if body.Domain == d {
-			domainAllowed = true
-			break
-		}
-	}
-	if !domainAllowed {
+	if !slices.Contains(allowedAliasDomains, body.Domain) {
 		http.Error(w, "Domain not allowed", http.StatusBadRequest)
 		return
 	}