Brand consistency: emails say "upbeatBytes" (From + digest body)

Per the brand-name standard (camelCase, one word). Updated the SMTP From default and
the digest email body/subject strings. Live env From values (auth.env + goodnews.env)
updated to match. (Web/OG brand strings in share.py + app.html are the remaining sweep.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_digest.py

@@ -20,10 +20,10 @@ def _seed(c, n=5, date="2026-06-09"):
 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 "Tuesday's upbeatBytes" in subject and "1 calm read" in subject
     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 "Good thing" in html and "Read on upbeatBytes" in html and "Unsubscribe" in html
     assert "you missed" not in (text + html).lower()  # no guilt language