feat(auth): add Microsoft OAuth callback

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

backend/app/api/endpoints/oauth.py

@@ -104,3 +104,20 @@ async def google_callback(
         refresh_token=create_refresh_token({"sub": str(user.id)}),
         is_new_user=is_new,
     )
+
+
+@router.post("/microsoft/callback", response_model=OAuthCallbackResponse)
+async def microsoft_callback(
+    payload: OAuthCallbackPayload,
+    db: Annotated[AsyncSession, Depends(get_admin_db)],
+) -> OAuthCallbackResponse:
+    if not settings.MS_CLIENT_ID:
+        raise HTTPException(status_code=503, detail="Microsoft sign-in not configured")
+    redirect_uri = f"{settings.OAUTH_REDIRECT_BASE}/auth/microsoft/callback"
+    profile = await microsoft_exchange_code(payload.code, redirect_uri)
+    user, is_new = await _sign_in_or_register(db, "microsoft", profile)
+    return OAuthCallbackResponse(
+        access_token=create_access_token({"sub": str(user.id)}),
+        refresh_token=create_refresh_token({"sub": str(user.id)}),
+        is_new_user=is_new,
+    )