settings: PATCH /api/settings/language with ACTIVE_LANGUAGES gate

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/routers/api.py

@@ -21,6 +21,7 @@ import httpx
 from pydantic import BaseModel, Field
 
 from app.auth import require_token, maybe_current_user, CurrentUser
+from app.services.i18n import ACTIVE_LANGUAGES
 from app.config import get_settings
 from app.db import get_session, utcnow
 from app.services.openrouter import (
@@ -895,3 +896,38 @@ async def patch_digest_prefs(
     user.digest_tone = payload.tone
     await session.commit()
     return DigestPrefsOut(opt_in=payload.opt_in, tone=payload.tone)
+
+
+# ---------------------------------------------------------------------------
+# Settings — language preference
+# ---------------------------------------------------------------------------
+
+
+class LanguagePrefsIn(BaseModel):
+    lang: str
+
+
+class LanguagePrefsOut(BaseModel):
+    lang: str
+
+
+@router.patch("/settings/language", response_model=LanguagePrefsOut)
+async def patch_language_prefs(
+    payload: LanguagePrefsIn,
+    principal: CurrentUser = Depends(require_token),
+    session: AsyncSession = Depends(get_session),
+) -> LanguagePrefsOut:
+    if principal.user is None:
+        raise HTTPException(status_code=400, detail="no_user_context")
+    lang = (payload.lang or "").strip().lower()
+    if lang not in ACTIVE_LANGUAGES:
+        raise HTTPException(
+            status_code=400,
+            detail=f"unsupported language: {payload.lang!r}",
+        )
+    user = await session.get(User, principal.user.id)
+    if user is None:
+        raise HTTPException(status_code=404, detail="user_not_found")
+    user.lang = lang
+    await session.commit()
+    return LanguagePrefsOut(lang=lang)