fix(qa): enforce critical-QA gate on export + fix neutral_background critical-but-passed (GAP-15/16, INV-QA3/EX3)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/qa_validator.py

@@ -67,7 +67,7 @@ def check_neutral_background(blocks: list[dict]) -> dict:
     """בדיקת ניטרליות בלוק הרקע (ו)."""
     vav = next((b for b in blocks if b["block_id"] == "block-vav"), None)
     if not vav or not vav.get("content"):
-        return {"name": "neutral_background", "passed": True, "errors": [], "severity": "critical"}
+        return {"name": "neutral_background", "passed": True, "errors": [], "severity": "warning"}
 
     errors = []
     lines = vav["content"].split("\n")