fix(lint): restore noqa directives and register custom codes with ruff

Reverts the noqa: removal — the architecture validators (SVC-006,
SEC-034, MOD-004, API-007) use these to skip known-safe violations.
Added ruff lint.external config so ruff treats them as valid codes.

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

app/modules/tenancy/models/merchant_domain.py

@@ -103,7 +103,7 @@ class MerchantDomain(Base, TimestampMixin):
         - EXAMPLE.COM -> example.com
         """
         # Remove protocol
-        domain = domain.replace("https://", "").replace("http://", "")  # SEC-034
+        domain = domain.replace("https://", "").replace("http://", "")  # noqa: SEC-034
 
         # Remove trailing slash
         domain = domain.rstrip("/")

app/modules/tenancy/models/store_domain.py

@@ -92,7 +92,7 @@ class StoreDomain(Base, TimestampMixin):
         - EXAMPLE.COM -> example.com
         """
         # Remove protocol
-        domain = domain.replace("https://", "").replace("http://", "")  # SEC-034
+        domain = domain.replace("https://", "").replace("http://", "")  # noqa: SEC-034
 
         # Remove trailing slash
         domain = domain.rstrip("/")