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/availability/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";
@@ -25,7 +25,6 @@ import {
 const { slotDurationMinutes, maxAdvanceDays, minAdvanceDays } = constants.exchange;
 
 type _AvailabilityDay = components["schemas"]["AvailabilityDay"];
-type AvailabilityResponse = components["schemas"]["AvailabilityResponse"];
 type TimeSlot = components["schemas"]["TimeSlot"];
 
 // Generate time options for dropdowns (15-min intervals)
@@ -73,9 +72,7 @@ export default function AdminAvailabilityPage() {
     try {
       const fromDate = formatDate(dateRange[0]);
       const toDate = formatDate(dateRange[dateRange.length - 1]);
-      const data = await api.get<AvailabilityResponse>(
-        `/api/admin/availability?from=${fromDate}&to=${toDate}`
-      );
+      const data = await adminApi.getAvailability(fromDate, toDate);
 
       const map = new Map<string, TimeSlot[]>();
       for (const day of data.days) {
@@ -140,7 +137,7 @@ export default function AdminAvailabilityPage() {
         end_time: s.end_time + ":00",
       }));
 
-      await api.put("/api/admin/availability", {
+      await adminApi.updateAvailability({
         date: formatDate(selectedDate),
         slots,
       });
@@ -161,7 +158,7 @@ export default function AdminAvailabilityPage() {
     setError(null);
 
     try {
-      await api.put("/api/admin/availability", {
+      await adminApi.updateAvailability({
         date: formatDate(selectedDate),
         slots: [],
       });
@@ -203,7 +200,7 @@ export default function AdminAvailabilityPage() {
     setError(null);
 
     try {
-      await api.post("/api/admin/availability/copy", {
+      await adminApi.copyAvailability({
         source_date: copySource,
         target_dates: Array.from(copyTargets),
       });

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) {

frontend/app/admin/price-history/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 { sharedStyles } from "../../styles/shared";
 import { Header } from "../../components/Header";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
@@ -24,7 +24,7 @@ export default function AdminPriceHistoryPage() {
     setError(null);
     setIsLoadingData(true);
     try {
-      const data = await api.get<PriceHistoryRecord[]>("/api/audit/price-history");
+      const data = await adminApi.getPriceHistory();
       setRecords(data);
     } catch (err) {
       setRecords([]);
@@ -38,7 +38,7 @@ export default function AdminPriceHistoryPage() {
     setIsFetching(true);
     setError(null);
     try {
-      await api.post<PriceHistoryRecord>("/api/audit/price-history/fetch", {});
+      await adminApi.fetchPrice();
       await fetchRecords();
     } catch (err) {
       setError(err instanceof Error ? err.message : "Failed to fetch price");

frontend/app/admin/trades/page.tsx

@@ -2,7 +2,7 @@
 
 import { useEffect, useState, useCallback, CSSProperties } from "react";
 import { Permission } from "../../auth-context";
-import { api } from "../../api";
+import { adminApi } from "../../api";
 import { Header } from "../../components/Header";
 import { SatsDisplay } from "../../components/SatsDisplay";
 import { useRequireAuth } from "../../hooks/useRequireAuth";
@@ -47,7 +47,7 @@ export default function AdminTradesPage() {
 
   const fetchUpcomingTrades = useCallback(async (): Promise<string | null> => {
     try {
-      const data = await api.get<AdminExchangeResponse[]>("/api/admin/trades/upcoming");
+      const data = await adminApi.getUpcomingTrades();
       setUpcomingTrades(data);
       return null;
     } catch (err) {
@@ -58,21 +58,17 @@ export default function AdminTradesPage() {
 
   const fetchPastTrades = useCallback(async (): Promise<string | null> => {
     try {
-      let url = "/api/admin/trades/past";
-      const params = new URLSearchParams();
-
+      const params: { status?: string; user_search?: string } = {};
       if (statusFilter !== "all") {
-        params.append("status", statusFilter);
+        params.status = statusFilter;
       }
       if (userSearch.trim()) {
-        params.append("user_search", userSearch.trim());
+        params.user_search = userSearch.trim();
       }
 
-      if (params.toString()) {
-        url += `?${params.toString()}`;
-      }
-
-      const data = await api.get<AdminExchangeResponse[]>(url);
+      const data = await adminApi.getPastTrades(
+        Object.keys(params).length > 0 ? params : undefined
+      );
       setPastTrades(data);
       return null;
     } catch (err) {
@@ -105,12 +101,13 @@ export default function AdminTradesPage() {
     setError(null);
 
     try {
-      const endpoint =
-        action === "no_show"
-          ? `/api/admin/trades/${publicId}/no-show`
-          : `/api/admin/trades/${publicId}/${action}`;
-
-      await api.post<AdminExchangeResponse>(endpoint, {});
+      if (action === "complete") {
+        await adminApi.completeTrade(publicId);
+      } else if (action === "no_show") {
+        await adminApi.noShowTrade(publicId);
+      } else if (action === "cancel") {
+        await adminApi.cancelTrade(publicId);
+      }
       // Refetch trades - errors from fetch are informational, not critical
       const [upcomingErr, pastErr] = await Promise.all([fetchUpcomingTrades(), fetchPastTrades()]);
       const fetchErrors = [upcomingErr, pastErr].filter(Boolean);