perf: convert sync file I/O to async in API routes (P0-2)

Replace all readFileSync/existsSync/readdirSync with fs.promises
equivalents to unblock the Node event loop under concurrency.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/files/browse/route.ts

@@ -1,16 +1,16 @@
 import { NextRequest, NextResponse } from 'next/server';
-import fs from 'fs';
+import fs from 'fs/promises';
 import path from 'path';
 import os from 'os';
 import type { ErrorResponse } from '@/types';
 
-function getWindowsDrives(): string[] {
+async function getWindowsDrives(): Promise<string[]> {
   if (process.platform !== 'win32') return [];
   const drives: string[] = [];
   for (let i = 65; i <= 90; i++) {
     const drive = String.fromCharCode(i) + ':\\';
     try {
-      fs.accessSync(drive);
+      await fs.access(drive);
       drives.push(drive);
     } catch {
       // drive not available
@@ -26,7 +26,9 @@ export async function GET(request: NextRequest) {
 
   const resolvedDir = path.resolve(dir);
 
-  if (!fs.existsSync(resolvedDir)) {
+  try {
+    await fs.access(resolvedDir);
+  } catch {
     return NextResponse.json<ErrorResponse>(
       { error: 'Directory does not exist' },
       { status: 404 }
@@ -34,7 +36,7 @@ export async function GET(request: NextRequest) {
   }
 
   try {
-    const entries = fs.readdirSync(resolvedDir, { withFileTypes: true });
+    const entries = await fs.readdir(resolvedDir, { withFileTypes: true });
     const directories = entries
       .filter((e) => e.isDirectory() && !e.name.startsWith('.'))
       .map((e) => ({
@@ -43,7 +45,7 @@ export async function GET(request: NextRequest) {
       }))
       .sort((a, b) => a.name.localeCompare(b.name));
 
-    const drives = getWindowsDrives();
+    const drives = await getWindowsDrives();
 
     return NextResponse.json({
       current: resolvedDir,

src/app/api/files/preview/route.ts

@@ -1,4 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
+import path from 'path';
+import os from 'os';
 import { readFilePreview, isPathSafe } from '@/lib/files';
 import type { FilePreviewResponse, ErrorResponse } from '@/types';
 
@@ -14,8 +16,6 @@ export async function GET(request: NextRequest) {
     );
   }
 
-  const path = require('path');
-  const os = require('os');
   const resolvedPath = path.resolve(filePath);
   const homeDir = os.homedir();
 
@@ -52,7 +52,7 @@ export async function GET(request: NextRequest) {
   }
 
   try {
-    const preview = readFilePreview(resolvedPath, Math.min(maxLines, 1000));
+    const preview = await readFilePreview(resolvedPath, Math.min(maxLines, 1000));
     return NextResponse.json<FilePreviewResponse>({ preview });
   } catch (error) {
     return NextResponse.json<ErrorResponse>(

src/app/api/files/raw/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest } from 'next/server';
-import fs from 'fs';
+import fs from 'fs/promises';
 import path from 'path';
 import os from 'os';
 import { isPathSafe } from '@/lib/files';
@@ -104,14 +104,16 @@ export async function GET(request: NextRequest) {
     });
   }
 
-  if (!fs.existsSync(resolved)) {
+  try {
+    await fs.access(resolved);
+  } catch {
     return new Response(JSON.stringify({ error: 'File not found' }), {
       status: 404,
       headers: { 'Content-Type': 'application/json' },
     });
   }
 
-  const stat = fs.statSync(resolved);
+  const stat = await fs.stat(resolved);
   if (!stat.isFile()) {
     return new Response(JSON.stringify({ error: 'Not a file' }), {
       status: 400,
@@ -119,7 +121,7 @@ export async function GET(request: NextRequest) {
     });
   }
 
-  const buffer = fs.readFileSync(resolved);
+  const buffer = await fs.readFile(resolved);
   const ext = path.extname(resolved).toLowerCase();
   const contentType = MIME_TYPES[ext] || 'application/octet-stream';
 

src/app/api/files/route.ts

@@ -1,4 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server';
+import path from 'path';
+import os from 'os';
 import { scanDirectory, isPathSafe } from '@/lib/files';
 import type { FileTreeResponse, ErrorResponse } from '@/types';
 
@@ -14,8 +16,6 @@ export async function GET(request: NextRequest) {
     );
   }
 
-  const path = require('path');
-  const os = require('os');
   const resolvedDir = path.resolve(dir);
   const homeDir = os.homedir();
 
@@ -52,7 +52,7 @@ export async function GET(request: NextRequest) {
   }
 
   try {
-    const tree = scanDirectory(resolvedDir, Math.min(depth, 5));
+    const tree = await scanDirectory(resolvedDir, Math.min(depth, 5));
     return NextResponse.json<FileTreeResponse>({ tree, root: resolvedDir });
   } catch (error) {
     return NextResponse.json<ErrorResponse>(

src/app/api/uploads/route.ts

@@ -1,5 +1,5 @@
 import { NextRequest } from 'next/server';
-import fs from 'fs';
+import fs from 'fs/promises';
 import path from 'path';
 
 export const runtime = 'nodejs';
@@ -42,14 +42,16 @@ export async function GET(request: NextRequest) {
     });
   }
 
-  if (!fs.existsSync(resolved)) {
+  try {
+    await fs.access(resolved);
+  } catch {
     return new Response(JSON.stringify({ error: 'File not found' }), {
       status: 404,
       headers: { 'Content-Type': 'application/json' },
     });
   }
 
-  const buffer = fs.readFileSync(resolved);
+  const buffer = await fs.readFile(resolved);
   const ext = path.extname(resolved).toLowerCase();
   const contentType = MIME_TYPES[ext] || 'application/octet-stream';