fix(insta-render): _build_pages tolerates dict/list from NAS API

NAS GET /api/insta/slates/{id}는 cover_copy/body_copies/cta_copy를
이미 dict/list로 parse해서 반환 (main.py:193-198). 워커가 json.loads(dict)
시도하다 TypeError로 즉시 fail.

_coerce 헬퍼로 string / dict-list 둘 다 처리하도록 보완.
3 unit tests PASS (영향 없음).

Plan-B-Insta T15 fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/insta-render/card_renderer.py

@@ -77,11 +77,24 @@ def _env() -> Environment:
     )
 
 
+def _coerce(value, default):
+    """NAS API는 cover_copy/body_copies/cta_copy를 dict/list로 반환하지만
+    구버전 호환을 위해 JSON 문자열도 처리."""
+    if value is None or value == "":
+        return default
+    if isinstance(value, str):
+        try:
+            return json.loads(value)
+        except (ValueError, TypeError):
+            return default
+    return value
+
+
 def _build_pages(slate: dict) -> List[dict]:
     """slate dict → 10 page specs."""
-    cover = json.loads(slate["cover_copy"] or "{}")
+    cover = _coerce(slate.get("cover_copy"), {})
-    bodies = json.loads(slate["body_copies"] or "[]")
+    bodies = _coerce(slate.get("body_copies"), [])
-    cta = json.loads(slate["cta_copy"] or "{}")
+    cta = _coerce(slate.get("cta_copy"), {})
     accent = cover.get("accent_color") or "#0F62FE"
     pages: List[dict] = []
     pages.append({