feat: detect subscriber-only tweets via tweetInterstitial (#33)

Extract visibility metadata from TweetWithVisibilityResults wrapper
before unwrapping. Adds is_subscriber_only field to Tweet model,
with full serialization roundtrip and test coverage.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -38,6 +38,7 @@ def tweet_factory():
             score=overrides.pop("score", 0.0),
             article_title=overrides.pop("article_title", None),
             article_text=overrides.pop("article_text", None),
+            is_subscriber_only=overrides.pop("is_subscriber_only", False),
         )
 
     return _make_tweet