Add i18n support with English and Spanish translations

- Created i18n package with translator and middleware
- Added translation files for English (en.json) and Spanish (es.json)
- Updated all HTTP handlers to use i18n for error/success messages
- Added comprehensive test coverage for i18n (87%)
- Updated CI workflow to use Go 1.24
- All tests passing with 50.8% total coverage

internal/infrastructure/http/i18n_helper.go

@@ -0,0 +1,52 @@
+package http
+
+import (
+	"net/http"
+	"strings"
+
+	"apocapoc-api/internal/i18n"
+
+	"golang.org/x/text/language"
+)
+
+func respondErrorI18n(w http.ResponseWriter, r *http.Request, translator *i18n.Translator, status int, key string) {
+	lang := i18n.GetLanguageFromContext(r.Context())
+	message := translator.Error(lang, key)
+	respondJSON(w, status, ErrorResponse{Error: message})
+}
+
+func respondSuccessI18n(w http.ResponseWriter, r *http.Request, translator *i18n.Translator, key string) {
+	lang := i18n.GetLanguageFromContext(r.Context())
+	message := translator.Success(lang, key)
+	respondJSON(w, http.StatusOK, map[string]string{"message": message})
+}
+
+func respondValidationErrorI18n(w http.ResponseWriter, r *http.Request, translator *i18n.Translator, err error) {
+	lang := i18n.GetLanguageFromContext(r.Context())
+	errMsg := err.Error()
+	var field string
+	var translatedMsg string
+
+	if strings.Contains(errMsg, ": ") {
+		parts := strings.SplitN(errMsg, ": ", 3)
+		if len(parts) >= 3 {
+			field = parts[1]
+			validationKey := parts[2]
+			translatedMsg = translator.Validation(lang, validationKey)
+			respondJSON(w, http.StatusBadRequest, ValidationErrorResponse{
+				Error: translatedMsg,
+				Field: field,
+			})
+			return
+		}
+	}
+
+	respondJSON(w, http.StatusBadRequest, ErrorResponse{
+		Error: errMsg,
+	})
+}
+
+func getLanguageFromRequest(r *http.Request, translator *i18n.Translator) language.Tag {
+	lang := i18n.GetLanguageFromContext(r.Context())
+	return lang
+}