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>

goodnews/digest.py

@@ -148,9 +148,12 @@ def send_due_digests(conn: sqlite3.Connection, force: bool = False, base: str |
             conn.execute("UPDATE users SET digest_unsub_token = ? WHERE id = ?", (token, user["id"]))
             conn.commit()
             user["digest_unsub_token"] = token
-        subject, text, html = build_digest(items, brief_date, unsub_url(user, base), base)
+        link = unsub_url(user, base)
+        subject, text, html = build_digest(items, brief_date, link, base)
+        # RFC 2369 / 8058: let inboxes offer native one-click unsubscribe.
+        headers = {"List-Unsubscribe": f"<{link}>", "List-Unsubscribe-Post": "List-Unsubscribe=One-Click"}
         try:
-            email_send.send_email(user["email"], subject, text, html=html)
+            email_send.send_email(user["email"], subject, text, html=html, headers=headers)
         except Exception:  # noqa: BLE001 — one bad send shouldn't stop the rest; retry next window
             continue
         conn.execute(