feat: Add API client library with subscriber and system endpoints for web dashboard

- Add apiFetch helper with JSON handling and error parsing
- Add Subscriber, SystemStats, HealthStatus, PaginatedResponse interfaces
- Add subscribers CRUD methods: list, get, create, update, delete with pagination support
- Add system endpoints: stats and health status
- Configure API_BASE from NEXT_PUBLIC_API_URL environment variable with localhost fallback

apps/web-dashboard/src/lib/api.ts

@@ -0,0 +1,69 @@
+const API_BASE = process.env.NEXT_PUBLIC_API_URL || "http://localhost:8000";
+
+async function apiFetch<T>(path: string, init?: RequestInit): Promise<T> {
+  const res = await fetch(`${API_BASE}${path}`, {
+    headers: { "Content-Type": "application/json", ...init?.headers },
+    ...init,
+  });
+  if (!res.ok) {
+    const body = await res.json().catch(() => ({}));
+    throw new Error(body.message || res.statusText);
+  }
+  return res.json();
+}
+
+export interface Subscriber {
+  id: number;
+  imsi: string;
+  msisdn: string;
+  first_name: string;
+  last_name: string;
+  email: string;
+  organization_id: string;
+  status: "active" | "inactive" | "suspended" | "terminated" | "provisioning";
+  plan_id: number;
+  balance: number;
+  profile_status: string;
+  created_at: string;
+  updated_at: string;
+}
+
+export interface SystemStats {
+  active_sessions: number;
+  total_accounts: number;
+  blocked_users: number;
+  low_balance_alerts: number;
+  uptime: number;
+}
+
+export interface HealthStatus {
+  redis_connected: boolean;
+  active_sync: boolean;
+  last_sync: string;
+  memory_usage: number;
+}
+
+export interface PaginatedResponse<T> {
+  data: T[];
+  total: number;
+  page: number;
+  page_size: number;
+}
+
+export const api = {
+  subscribers: {
+    list: (page = 1, pageSize = 20) =>
+      apiFetch<PaginatedResponse<Subscriber>>(`/api/subscribers?page=${page}&pageSize=${pageSize}`),
+    get: (id: number) => apiFetch<Subscriber>(`/api/subscribers/${id}`),
+    create: (data: Partial<Subscriber>) =>
+      apiFetch<Subscriber>("/api/subscribers", { method: "POST", body: JSON.stringify(data) }),
+    update: (id: number, data: Partial<Subscriber>) =>
+      apiFetch<Subscriber>(`/api/subscribers/${id}`, { method: "PUT", body: JSON.stringify(data) }),
+    delete: (id: number) =>
+      apiFetch<void>(`/api/subscribers/${id}`, { method: "DELETE" }),
+  },
+  system: {
+    stats: () => apiFetch<SystemStats>("/api/system/stats"),
+    health: () => apiFetch<HealthStatus>("/api/system/health"),
+  },
+};