settings: digest opt-in + tone (PATCH /api/settings/digest + UI)

Adds DigestPrefsIn/Out models, PATCH /api/settings/digest endpoint, email
digest section in settings.html, and last_email_send context in pages.py.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/routers/pages.py

@@ -11,7 +11,7 @@ from sqlalchemy.ext.asyncio import AsyncSession
 from app.auth import CurrentUser, maybe_current_user, require_auth, require_token
 from app.config import get_settings, load_groups
 from app.db import get_session
-from app.models import Referral, StrategicLog, User
+from app.models import EmailSend, Referral, StrategicLog, User
 from app.services.access import paid_status
 from app.services.referral_service import assign_code_if_missing
 from app.templates_env import templates
@@ -147,6 +147,13 @@ async def settings_page(
 
     invite_url = str(request.url_for("login_page")) + f"?ref={user.referral_code}"
 
+    last_email_send = (await session.execute(
+        select(EmailSend)
+        .where(EmailSend.user_id == user.id)
+        .order_by(desc(EmailSend.sent_at))
+        .limit(1)
+    )).scalar_one_or_none()
+
     return templates.TemplateResponse(
         request, "settings.html",
         {
@@ -155,5 +162,6 @@ async def settings_page(
             "pending_count": int(pending_count),
             "converted_count": int(converted_count),
             "paid": paid_status(user),
+            "last_email_send": last_email_send,
         },
     )