feat(auth): add Microsoft OAuth callback

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

backend/tests/test_oauth_callbacks.py

@@ -93,3 +93,28 @@ async def test_google_callback_503_when_unconfigured(client, monkeypatch):
         "/api/v1/auth/google/callback", json={"code": "x"}
     )
     assert response.status_code == 503
+
+
+@pytest.mark.asyncio
+async def test_microsoft_callback_creates_user(client, test_db, monkeypatch):
+    from app.core.config import settings
+    monkeypatch.setattr(settings, "MS_CLIENT_ID", "client_dummy")
+    monkeypatch.setattr(settings, "MS_CLIENT_SECRET", "secret_dummy")
+
+    profile = OAuthProfile(
+        provider_subject="ms_subject_789",
+        email="msuser@example.com",
+        name="MS User",
+    )
+    with patch("app.api.endpoints.oauth.microsoft_exchange_code", return_value=profile):
+        response = await client.post(
+            "/api/v1/auth/microsoft/callback", json={"code": "auth_code"}
+        )
+    assert response.status_code == 200, response.json()
+    user = (await test_db.execute(
+        select(User).where(User.email == "msuser@example.com")
+    )).scalar_one()
+    identity = (await test_db.execute(
+        select(OAuthIdentity).where(OAuthIdentity.user_id == user.id)
+    )).scalar_one()
+    assert identity.provider == "microsoft"