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/api.py

@@ -528,16 +528,25 @@ def create_app() -> FastAPI:
             conn.commit()
         return {"ok": True, "digest_enabled": body.enabled}
 
-    @app.get("/api/digest/unsubscribe", response_class=HTMLResponse)
-    def digest_unsubscribe(u: int = Query(...), t: str = Query(...)) -> HTMLResponse:
-        # One-click, no login: match the per-user token, then turn the digest off.
-        ok = False
+    def _do_unsubscribe(u: int, t: str) -> bool:
         with get_conn() as conn:
             row = conn.execute("SELECT digest_unsub_token FROM users WHERE id = ?", (u,)).fetchone()
             if row and row["digest_unsub_token"] and hmac.compare_digest(row["digest_unsub_token"], t):
                 conn.execute("UPDATE users SET digest_enabled = 0 WHERE id = ?", (u,))
                 conn.commit()
-                ok = True
+                return True
+        return False
+
+    @app.post("/api/digest/unsubscribe")
+    def digest_unsubscribe_oneclick(u: int = Query(...), t: str = Query(...)) -> dict:
+        # RFC 8058 one-click: the mailbox provider POSTs here; just do it + 200.
+        _do_unsubscribe(u, t)
+        return {"ok": True}
+
+    @app.get("/api/digest/unsubscribe", response_class=HTMLResponse)
+    def digest_unsubscribe(u: int = Query(...), t: str = Query(...)) -> HTMLResponse:
+        # One-click, no login: match the per-user token, then turn the digest off.
+        ok = _do_unsubscribe(u, t)
         msg = (
             "You’re unsubscribed from the daily digest. No hard feelings — "
             "Upbeat Bytes is always here when you want it."