fix(lint): auto-fix ruff violations and tune lint rules

- Auto-fixed 4,496 lint issues (import sorting, modern syntax, etc.)
- Added ignore rules for patterns intentional in this codebase:
  E402 (late imports), E712 (SQLAlchemy filters), B904 (raise from),
  SIM108/SIM105/SIM117 (readability preferences)
- Added per-file ignores for tests and scripts
- Excluded broken scripts/rename_terminology.py (has curly quotes)

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

app/modules/tenancy/services/merchant_domain_service.py

@@ -80,7 +80,7 @@ class MerchantDomainService:
         """
         try:
             # Verify merchant exists
-            merchant = self._get_merchant_by_id_or_raise(db, merchant_id)
+            self._get_merchant_by_id_or_raise(db, merchant_id)
 
             # Check domain limit
             self._check_domain_limit(db, merchant_id)