feat: add QR code device pairing for mobile app login

app/api/auth/pair/redeem/route.ts

@@ -0,0 +1,36 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { logger } from '@/lib/logger';
+import { consumePairing } from '@/lib/auth/pairing-store';
+
+// Phone side of the cross-device QR login. The app POSTs the pairing code it
+// scanned; we hand back the OAuth token bundle the desktop stashed at
+// /api/auth/pair/create. The code is the only credential required — it is
+// high-entropy, single-use, and expires within ~2 minutes — so this route is
+// intentionally unauthenticated (the scanning device has no webmail cookies).
+export async function POST(request: NextRequest) {
+  try {
+    const { pairing_code: pairingCode } = await request.json().catch(() => ({}));
+    if (!pairingCode || typeof pairingCode !== 'string') {
+      return NextResponse.json({ error: 'Missing pairing code' }, { status: 400 });
+    }
+
+    const tokens = consumePairing(pairingCode);
+    if (!tokens) {
+      // Unknown, expired, or already redeemed — do not distinguish.
+      return NextResponse.json({ error: 'Invalid or expired pairing code' }, { status: 400 });
+    }
+
+    return NextResponse.json({
+      flow: 'oauth',
+      server_url: tokens.serverUrl,
+      access_token: tokens.accessToken,
+      ...(tokens.refreshToken ? { refresh_token: tokens.refreshToken } : {}),
+      ...(typeof tokens.expiresIn === 'number' ? { expires_in: tokens.expiresIn } : {}),
+      token_endpoint: tokens.tokenEndpoint,
+      client_id: tokens.clientId,
+    });
+  } catch (error) {
+    logger.error('Pair redeem error', { error: error instanceof Error ? error.message : 'Unknown error' });
+    return NextResponse.json({ error: 'Internal server error' }, { status: 500 });
+  }
+}