fix(bird_x): skip all-None engagement dicts

When a tweet has no engagement metrics, _first_of() returns None for
every key, producing {"likes": None, "reposts": None, ...}.  This
all-None dict propagates to signals.py where it is treated as "data
exists but is zero" rather than "no data available."  Return None
instead when every engagement field is missing.

scripts/lib/bird_x.py

@@ -460,7 +460,7 @@ def parse_bird_response(response: Dict[str, Any], query: str = "") -> List[Dict[
             "url": url,
             "author_handle": author_handle.lstrip("@"),
             "date": date,
-            "engagement": engagement,
+            "engagement": engagement if any(v is not None for v in engagement.values()) else None,
             "why_relevant": "",  # Bird doesn't provide relevance explanations
             "relevance": _compute_relevance(query, str(tweet.get("text", ""))) if query else 0.7,
         }