fix: Use container-internal port 80 in gateway Caddyfile

The Caddyfile was using the host port (e.g., 9100) as the Caddy listen
address, but Docker maps host:9100 → container:80. Caddy inside the
container was listening on 9100 while Docker only forwarded to port 80,
causing all health checks to fail with "connection reset by peer".

apps/server/preview/compose-generator.test.ts

@@ -164,7 +164,7 @@ describe('generateGatewayCaddyfile', () => {
 
     const caddyfile = generateGatewayCaddyfile(previews, 9100);
     expect(caddyfile).toContain('auto_https off');
-    expect(caddyfile).toContain('localhost:9100 {');
+    expect(caddyfile).toContain(':80 {');
     expect(caddyfile).toContain('handle_path /abc123/*');
     expect(caddyfile).toContain('reverse_proxy cw-preview-abc123-app:3000');
   });
@@ -193,7 +193,7 @@ describe('generateGatewayCaddyfile', () => {
     ]);
 
     const caddyfile = generateGatewayCaddyfile(previews, 9100);
-    expect(caddyfile).toContain('localhost:9100 {');
+    expect(caddyfile).toContain(':80 {');
     expect(caddyfile).toContain('handle_path /abc/*');
     expect(caddyfile).toContain('handle_path /xyz/*');
     expect(caddyfile).toContain('reverse_proxy cw-preview-abc-app:3000');

apps/server/preview/gateway.ts

@@ -201,14 +201,16 @@ export class GatewayManager {
  */
 export function generateGatewayCaddyfile(
   previews: Map<string, GatewayRoute[]>,
-  port: number,
+  _port: number,
 ): string {
+  // Caddy runs inside a container where Docker maps host:${port} → container:80.
+  // The Caddyfile must listen on the container-internal port (80), not the host port.
   const lines: string[] = [
     '{',
     '  auto_https off',
     '}',
     '',
-    `localhost:${port} {`,
+    `:80 {`,
   ];
 
   for (const [previewId, routes] of previews) {