fix(lint): pin ruff version, add pre-commit hook, fix all lint errors

- Pin ruff==0.8.4 in requirements-dev.txt (was >=0.8.4, CI got newer
  version with different import sorting rules)
- Add ruff to .pre-commit-config.yaml with --fix to auto-sort imports
  on commit (prevents PyCharm import reordering from reaching CI)
- Fix I001 import sorting in 6 files
- Fix F401 unused import (sqlalchemy.Numeric in subscription.py)
- Fix noqa false positive in validate_architecture.py comment

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

app/modules/billing/models/subscription.py

@@ -22,7 +22,6 @@ from sqlalchemy import (
     ForeignKey,
     Index,
     Integer,
-    Numeric,
     String,
     Text,
 )

app/modules/billing/schemas/__init__.py

@@ -21,8 +21,6 @@ from app.modules.billing.schemas.billing import (
     CancelRequest,
     CancelResponse,
     CategoryListResponse,
-    ChangeTierRequest as BillingChangeTierRequest,
-    ChangeTierResponse as BillingChangeTierResponse,
     # Checkout & Portal schemas
     CheckoutRequest,
     CheckoutResponse,
@@ -56,12 +54,18 @@ from app.modules.billing.schemas.billing import (
     SubscriptionTierListResponse,
     SubscriptionTierResponse,
     SubscriptionTierUpdate,
-    TierListResponse,
-    TierResponse,
     # Subscription Tier Admin schemas
     TierFeatureLimitEntry,
+    TierListResponse,
+    TierResponse,
     UpcomingInvoiceResponse,
 )
+from app.modules.billing.schemas.billing import (
+    ChangeTierRequest as BillingChangeTierRequest,
+)
+from app.modules.billing.schemas.billing import (
+    ChangeTierResponse as BillingChangeTierResponse,
+)
 from app.modules.billing.schemas.subscription import (
     ChangeTierRequest,
     ChangeTierResponse,