Digest email: 'Daily Highlights' masthead + warm intro, point back to the site

Per user feedback: rename from 'Today's good news' (which implied the site was
done for the day) to a publication-style 'Upbeat Bytes — Daily Highlights'
masthead with a warm morning intro and a sign-off that links back to the site
('more good news is always waiting'). Adds 'why it's here' to the plain-text
part too. No images by design (lightweight, mail-client-safe).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_digest.py

@@ -21,7 +21,8 @@ def test_build_digest_is_calm_and_dated():
     items = [{"id": 1, "title": "Good thing", "canonical_url": "http://a/1", "source": "Src", "summary": "nice", "reason_text": "wonder", "paywalled": False}]
     subject, text, html = digest.build_digest(items, "2026-06-09", "http://ub/unsub")
     assert "Tuesday's Upbeat Bytes" in subject and "1 calm read" in subject
-    assert "You're caught up" in text and "http://ub/unsub" in text
+    assert "Daily Highlights" in text and "Daily Highlights" in html
+    assert "more good news is always" in text and "http://ub/unsub" in text  # points back to the site
     assert "Good thing" in html and "Read on Upbeat Bytes" in html and "Unsubscribe" in html
     assert "you missed" not in (text + html).lower()  # no guilt language