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/messaging/services/email_service.py

@@ -1336,7 +1336,7 @@ class EmailService:
                 related_id=related_id,
             )
             self.db.add(log)
-            self.db.commit()  # SVC-006 - Email logs are side effects, commit immediately
+            self.db.commit()  # noqa: SVC-006 - Email logs are side effects, commit immediately
             return log
 
         # Inject branding variables if requested
@@ -1471,7 +1471,7 @@ class EmailService:
         if not email_enabled:
             log.status = EmailStatus.FAILED.value
             log.error_message = "Email sending is disabled"
-            self.db.commit()  # SVC-006 - Email logs are side effects, commit immediately
+            self.db.commit()  # noqa: SVC-006 - Email logs are side effects, commit immediately
             logger.info(f"Email sending disabled, skipping: {to_email}")
             return log
 
@@ -1497,7 +1497,7 @@ class EmailService:
             log.mark_failed(error or "Unknown error")
             logger.error(f"Email failed to {to_email}: {error}")
 
-        self.db.commit()  # SVC-006 - Email logs are side effects, commit immediately
+        self.db.commit()  # noqa: SVC-006 - Email logs are side effects, commit immediately
         return log