feat(auth): add oauth_identities table for Google/Microsoft sign-in

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

backend/tests/test_oauth_identity_model.py

@@ -0,0 +1,39 @@
+import uuid
+
+import pytest
+from sqlalchemy import select
+
+from app.models.oauth_identity import OAuthIdentity
+
+
+@pytest.mark.asyncio
+async def test_oauth_identity_unique_provider_subject(test_db, test_user):
+    """Two rows with same provider+subject should violate uniqueness."""
+    user_id = uuid.UUID(test_user["user_data"]["id"])
+
+    row1 = OAuthIdentity(
+        user_id=user_id,
+        provider="google",
+        provider_subject="abc-123",
+        provider_email_at_link="alex@acmemsp.com",
+    )
+    test_db.add(row1)
+    await test_db.commit()
+
+    row2 = OAuthIdentity(
+        user_id=user_id,
+        provider="google",
+        provider_subject="abc-123",
+        provider_email_at_link="alex@acmemsp.com",
+    )
+    test_db.add(row2)
+    with pytest.raises(Exception):  # IntegrityError
+        await test_db.commit()
+    await test_db.rollback()
+
+    rows = (
+        await test_db.execute(
+            select(OAuthIdentity).where(OAuthIdentity.user_id == user_id)
+        )
+    ).scalars().all()
+    assert len(rows) == 1