fix(risk-monitor): drop an unused import and align migration 033 with its model

ruff F401 failed the deploy: `import pytest` in test_event_study.py outlived the
pytest.approx assertion it was added for.

Compiling the migration for Postgres while checking that turned up a second
defect worth fixing while the table is still empty. It created a unique
constraint *and* a plain index on effective_date, while the model declares
`unique=True, index=True` -- one unique index. Both enforce uniqueness, but the
pairing left a redundant second index on the column and a permanent diff for
autogenerate to keep trying to reconcile. Now renders byte-for-byte what the
model declares, matching RegimeSnapshot.date.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/unit/test_event_study.py

@@ -5,8 +5,6 @@ from __future__ import annotations
 from copy import deepcopy
 from datetime import date, timedelta
 
-import pytest
-
 from app.services.breadth_service import _breadth_from_closes, compute_divergence_series
 from app.services.event_study_service import (
     MIN_EVENTS_FOR_CONFIDENCE,