chore: add pre-commit ruff hook with auto-fix

- scripts/hooks/pre-commit: runs ruff --fix for root and web/ (matching CI)
  and re-stages any auto-fixed files so they land in the commit
- Makefile: add install-hooks target (run once after clone)
- pyproject.toml: exclude web/ from root ruff (web has its own config)
- Fix remaining import sort warnings caught by the new hook

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/tests/test_billing_helpers.py

@@ -2,10 +2,6 @@
 Unit tests for billing SQL helpers, feature/limit access, and plan determination.
 """
 import pytest
-from hypothesis import HealthCheck, given
-from hypothesis import settings as h_settings
-from hypothesis import strategies as st
-
 from beanflows.billing.routes import (
     can_access_feature,
     get_billing_customer,
@@ -18,6 +14,9 @@ from beanflows.billing.routes import (
     upsert_subscription,
 )
 from beanflows.core import config
+from hypothesis import HealthCheck, given
+from hypothesis import settings as h_settings
+from hypothesis import strategies as st
 
 # ════════════════════════════════════════════════════════════
 # get_subscription