Style transcript turns and bump style.css cache version

The renderer emits .transcript-turn and .transcript-speaker, neither of which
had any CSS, so speaker labels and turns rendered unstyled. Extends the
existing .speaker-label rule rather than duplicating it.

Bumps style.css to v=7 everywhere — the file changed, and pages referenced a
mix of v=5 and v=6, so some visitors would have kept a cached copy without the
new rules.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_episode_render.py

@@ -102,7 +102,7 @@ def test_all_asset_paths_are_root_absolute(sample_episode):
     html = render_episode_page(sample_episode, turns=[])
     assert 'href="css/' not in html
     assert 'src="js/' not in html
-    assert 'href="/css/style.css?v=6"' in html
+    assert re.search(r'href="/css/style\.css\?v=\d+"', html)
     assert 'src="/js/footer.js"' in html