Refactor API layer into structured domain-specific modules

- Created new api/ directory with domain-specific API modules:
  - api/client.ts: Base API client with error handling
  - api/auth.ts: Authentication endpoints
  - api/exchange.ts: Exchange/price endpoints
  - api/trades.ts: User trade endpoints
  - api/profile.ts: Profile management endpoints
  - api/invites.ts: Invite endpoints
  - api/admin.ts: Admin endpoints
  - api/index.ts: Centralized exports

- Migrated all API calls from ad-hoc api.get/post/put to typed domain APIs
- Updated all imports across codebase
- Fixed test mocks to use new API structure
- Fixed type issues in validation utilities
- Removed old api.ts file

Benefits:
- Type-safe endpoints (no more string typos)
- Centralized API surface (easy to discover endpoints)
- Better organization (domain-specific modules)
- Uses generated OpenAPI types automatically

frontend/app/admin/invites/page.tsx

@@ -2,7 +2,7 @@
 
 import { useEffect, useState, useCallback } from "react";
 import { Permission } from "../../auth-context";
-import { api } from "../../api";
+import { adminApi } from "../../api";
 import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
 import { components } from "../../generated/api";
@@ -41,7 +41,7 @@ export default function AdminInvitesPage() {
 
   const fetchUsers = useCallback(async () => {
     try {
-      const data = await api.get<UserOption[]>("/api/admin/users");
+      const data = await adminApi.getUsers();
       setUsers(data);
     } catch (err) {
       console.error("Failed to fetch users:", err);
@@ -51,11 +51,7 @@ export default function AdminInvitesPage() {
   const fetchInvites = useCallback(async (page: number, status: string) => {
     setError(null);
     try {
-      let url = `/api/admin/invites?page=${page}&per_page=10`;
-      if (status) {
-        url += `&status=${status}`;
-      }
-      const data = await api.get<PaginatedInvites>(url);
+      const data = await adminApi.getInvites(page, 10, status || undefined);
       setData(data);
     } catch (err) {
       setData(null);
@@ -80,7 +76,7 @@ export default function AdminInvitesPage() {
     setCreateError(null);
 
     try {
-      await api.post("/api/admin/invites", {
+      await adminApi.createInvite({
         godfather_id: parseInt(newGodfatherId),
       });
       setNewGodfatherId("");
@@ -95,7 +91,7 @@ export default function AdminInvitesPage() {
 
   const handleRevoke = async (inviteId: number) => {
     try {
-      await api.post(`/api/admin/invites/${inviteId}/revoke`);
+      await adminApi.revokeInvite(inviteId);
       setError(null);
       fetchInvites(page, statusFilter);
     } catch (err) {