fix(insta-render): fonts.ready 대기 + PNG 비어있음 검증 (렌더 known-issue 해결)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

services/insta-render/card_renderer.py

@@ -151,8 +151,11 @@ async def _render_slate_locked(slate: dict, slate_id: int, template: str) -> Lis
                 html_path = f.name
             try:
                 await page.goto(f"file://{html_path}", wait_until="networkidle")
+                await page.evaluate("document.fonts.ready")   # 웹폰트 로딩 완료까지 대기
                 out_path = os.path.join(out_dir, f"{spec['page_no']:02d}.png")
                 await page.screenshot(path=out_path, full_page=False, omit_background=False)
+                if os.path.getsize(out_path) < 1000:          # 빈/깨진 PNG 방어
+                    raise RuntimeError(f"rendered PNG too small: {out_path}")
                 paths.append(out_path)
             finally:
                 try: