Digest polish: honest on-site wording, one-tap opt-in after sign-in, List-Unsubscribe

* On-site end-cap now says "You're caught up for now." — honest, since Highlights
  refreshes through the day (the email keeps the daily "see you tomorrow").
* Anonymous "Get tomorrow's brief by email" now honors the one-tap promise:
  sets a pending flag, opens sign-in, and auto-enables once auth resolves.
* Email compliance (RFC 2369/8058): send_email takes optional headers; the digest
  sets List-Unsubscribe + List-Unsubscribe-Post=One-Click, and a POST
  /api/digest/unsubscribe handles native one-click (GET still serves the page).

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

tests/test_digest.py

@@ -28,13 +28,16 @@ def test_build_digest_is_calm_and_dated():
 
 def test_send_due_digests_sends_dedupes_and_skips(monkeypatch, tmp_path):
     sent = []
-    monkeypatch.setattr(email_send, "send_email", lambda to, subj, text, html=None: sent.append(to))
+    monkeypatch.setattr(email_send, "send_email",
+                        lambda to, subj, text, html=None, headers=None, **k: sent.append((to, headers)))
     c = connect(str(tmp_path / "d.db")); init_db(c)
     date = _seed(c, n=5)
     monkeypatch.setattr(digest, "local_today", lambda: date)
     # force=True bypasses the morning window
     assert digest.send_due_digests(c, force=True) == 1
-    assert sent == ["reader@x.com"]
+    assert sent[0][0] == "reader@x.com"
+    hdrs = sent[0][1]  # RFC 8058 one-click unsubscribe headers present
+    assert "List-Unsubscribe" in hdrs and hdrs["List-Unsubscribe-Post"] == "List-Unsubscribe=One-Click"
     assert c.execute("SELECT item_count FROM digest_sends WHERE user_id=1").fetchone()[0] == 5
     # dedupe: a second run sends nothing
     assert digest.send_due_digests(c, force=True) == 0