test: add unit tests for untested modules

Add pytest infrastructure (pyproject.toml, conftest.py) and unit tests
for modules that previously had zero test coverage:

- test_schema_roundtrip.py: to_dict() serialization for all data classes
- test_reddit_enrich.py: URL parsing, thread data parsing, comment filtering
- test_reddit_sc.py: ScrapeCreators Reddit search (query expansion, subreddit discovery)
- test_instagram_sc.py: Instagram relevance scoring, tokenization, depth config

Includes fixtures/reddit_thread_sample.json for reddit_enrich tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_schema_roundtrip.py

@@ -0,0 +1,131 @@
+"""Tests for schema.py — data class serialization roundtrips."""
+
+from lib import schema
+
+
+class TestEngagement:
+    """Tests for Engagement.to_dict()."""
+
+    def test_sparse_fields(self):
+        eng = schema.Engagement(score=100, num_comments=50)
+        d = eng.to_dict()
+        assert d == {"score": 100, "num_comments": 50}
+        assert "likes" not in d
+
+    def test_all_none_returns_none(self):
+        eng = schema.Engagement()
+        assert eng.to_dict() is None
+
+    def test_all_fields(self):
+        eng = schema.Engagement(
+            score=1, num_comments=2, upvote_ratio=0.9,
+            likes=3, reposts=4, replies=5, quotes=6,
+            views=7, shares=8, volume=9.0, liquidity=10.0,
+        )
+        d = eng.to_dict()
+        assert len(d) == 11
+
+
+class TestComment:
+    def test_basic(self):
+        c = schema.Comment(score=50, date="2026-03-01", author="user", excerpt="text", url="http://x")
+        d = c.to_dict()
+        assert d["score"] == 50
+        assert d["author"] == "user"
+        assert len(d) == 5
+
+
+class TestRedditItem:
+    def test_roundtrip(self):
+        item = schema.RedditItem(
+            id="R1", title="Test", url="http://reddit.com/r/test",
+            subreddit="test", date="2026-03-01",
+            engagement=schema.Engagement(score=100),
+        )
+        d = item.to_dict()
+        assert d["id"] == "R1"
+        assert d["subreddit"] == "test"
+        assert d["engagement"] == {"score": 100}
+        assert "cross_refs" not in d
+
+    def test_cross_refs_included_when_present(self):
+        item = schema.RedditItem(
+            id="R1", title="T", url="u", subreddit="s",
+            cross_refs=["X1", "HN2"],
+        )
+        d = item.to_dict()
+        assert d["cross_refs"] == ["X1", "HN2"]
+
+
+class TestXItem:
+    def test_roundtrip(self):
+        item = schema.XItem(
+            id="X1", text="tweet", url="http://x.com/1",
+            author_handle="user", date="2026-03-01",
+        )
+        d = item.to_dict()
+        assert d["id"] == "X1"
+        assert d["author_handle"] == "user"
+        assert "cross_refs" not in d
+
+
+class TestYouTubeItem:
+    def test_roundtrip(self):
+        item = schema.YouTubeItem(
+            id="YT1", title="Video", url="http://youtube.com/1",
+            channel_name="chan",
+        )
+        d = item.to_dict()
+        assert d["channel_name"] == "chan"
+        assert d["date_confidence"] == "high"
+
+
+class TestTikTokItem:
+    def test_roundtrip(self):
+        item = schema.TikTokItem(
+            id="TK1", text="caption", url="http://tiktok.com/1",
+            author_name="creator", hashtags=["ai", "code"],
+        )
+        d = item.to_dict()
+        assert d["hashtags"] == ["ai", "code"]
+        assert d["author_name"] == "creator"
+
+
+class TestInstagramItem:
+    def test_roundtrip(self):
+        item = schema.InstagramItem(
+            id="IG1", text="caption", url="http://instagram.com/reel/1",
+            author_name="creator",
+        )
+        d = item.to_dict()
+        assert d["id"] == "IG1"
+
+
+class TestWebSearchItem:
+    def test_roundtrip(self):
+        item = schema.WebSearchItem(
+            id="W1", title="Article", url="http://example.com",
+            source_domain="example.com", snippet="text",
+        )
+        d = item.to_dict()
+        assert d["source_domain"] == "example.com"
+
+
+class TestHackerNewsItem:
+    def test_roundtrip(self):
+        item = schema.HackerNewsItem(
+            id="HN1", title="Show HN", url="http://example.com",
+            hn_url="http://news.ycombinator.com/item?id=1", author="pg",
+        )
+        d = item.to_dict()
+        assert d["hn_url"].startswith("http://news.ycombinator.com")
+
+
+class TestPolymarketItem:
+    def test_roundtrip(self):
+        item = schema.PolymarketItem(
+            id="PM1", title="Election", question="Who wins?",
+            url="http://polymarket.com/1",
+        )
+        d = item.to_dict()
+        assert d["question"] == "Who wins?"