fix(insta-lab): 선별 zero-pref 크래시 가드 + judge max_tokens 상향 + 404 테스트

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

insta-lab/app/selection.py

@@ -55,6 +55,8 @@ def score_candidates(
     w = weights or DEFAULT_WEIGHTS
     now = _parse_iso(now_iso) if now_iso else datetime.now(timezone.utc)
     max_w = max(prefs.values()) if prefs else 1.0
+    if max_w <= 0:
+        max_w = 1.0
     out: List[Dict[str, Any]] = []
     for c in candidates:
         cat = c.get("category", "")