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/api.py

@@ -8,6 +8,7 @@ from __future__ import annotations
 import calendar as _cal
 import re
 from datetime import date, datetime, timedelta, timezone
+from typing import Literal
 
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Query, Request, UploadFile
 from fastapi.responses import HTMLResponse, JSONResponse
@@ -809,3 +810,33 @@ async def chat(
         "prompt_tokens": result.prompt_tokens,
         "completion_tokens": result.completion_tokens,
     }
+
+
+# ---------------------------------------------------------------------------
+# Settings — digest preferences
+# ---------------------------------------------------------------------------
+
+
+class DigestPrefsIn(BaseModel):
+    opt_in: bool
+    tone: Literal["NOVICE", "INTERMEDIATE"]
+
+
+class DigestPrefsOut(BaseModel):
+    opt_in: bool
+    tone: str
+
+
+@router.patch("/settings/digest", response_model=DigestPrefsOut)
+async def patch_digest_prefs(
+    payload: DigestPrefsIn,
+    principal: CurrentUser = Depends(require_token),
+    session: AsyncSession = Depends(get_session),
+) -> DigestPrefsOut:
+    if principal.user is None:
+        # Admin bearer-token path — no per-user row to persist to.
+        raise HTTPException(status_code=400, detail="no_user_context")
+    principal.user.email_digest_opt_in = payload.opt_in
+    principal.user.digest_tone = payload.tone
+    await session.commit()
+    return DigestPrefsOut(opt_in=payload.opt_in, tone=payload.tone)