feat(i18n): implement dynamic system wording management

Add database schema and API endpoints for system wording keys and values
Replace hardcoded translations in frontend and backend with dynamic messages
Add System Wordings management interface to Admin view

frontend/src/services/api.ts

@@ -15,6 +15,21 @@ export interface Language {
   sortOrder: number;
 }
 
+export type SystemWordingSurface = 'frontend' | 'backend' | 'email';
+
+export interface SystemWording {
+  key: string;
+  module: string;
+  surface: SystemWordingSurface;
+  description: string;
+  placeholders: string[];
+  value: string;
+  defaultValue: string;
+  missing: boolean;
+  updatedAt: string | null;
+  updatedBy: UserSummary | null;
+}
+
 export interface NamedEntity {
   id: number;
   name: string;
@@ -508,6 +523,10 @@ export const api = {
     sendJson<Language[]>(`/api/admin/languages/${code}`, 'PUT', payload),
   reorderLanguages: (codes: string[]) => sendJson<Language[]>('/api/admin/languages/order', 'PUT', { codes }),
   deleteLanguage: (code: string) => deleteJson(`/api/admin/languages/${code}`),
+  systemWordings: (params: { locale?: string; module?: string; surface?: string; missing?: string } = {}) =>
+    getJson<SystemWording[]>(`/api/admin/system-wordings${buildQuery(params)}`),
+  updateSystemWording: (key: string, payload: { locale: string; value: string }) =>
+    sendJson<SystemWording[]>(`/api/admin/system-wordings/${encodeURIComponent(key)}`, 'PUT', payload),
   register: (payload: RegisterPayload) => sendJson<{ message: string }>('/api/auth/register', 'POST', payload),
   verifyEmail: (token: string) =>
     sendJson<{ message: string; user: AuthUser }>('/api/auth/verify-email', 'POST', { token }),