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/schemas/merchant_domain.py

@@ -34,7 +34,7 @@ class MerchantDomainCreate(BaseModel):
     def validate_domain(cls, v: str) -> str:
         """Validate and normalize domain."""
         # Remove protocol if present
-        domain = v.replace("https://", "").replace("http://", "")  # SEC-034
+        domain = v.replace("https://", "").replace("http://", "")  # noqa: SEC-034
 
         # Remove trailing slash
         domain = domain.rstrip("/")

app/modules/tenancy/schemas/store_domain.py

@@ -35,7 +35,7 @@ class StoreDomainCreate(BaseModel):
     def validate_domain(cls, v: str) -> str:
         """Validate and normalize domain."""
         # Remove protocol if present
-        domain = v.replace("https://", "").replace("http://", "")  # SEC-034
+        domain = v.replace("https://", "").replace("http://", "")  # noqa: SEC-034
 
         # Remove trailing slash
         domain = domain.rstrip("/")