feat(server): register flow flags accounts from test-seed invites

When a user registers with an invite_code whose marks_as_test=TRUE,
their users_v2.is_test_account is set to TRUE. Normal invite codes
and invite-less signups are unaffected.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/services/auth_service.py

@@ -101,6 +101,7 @@ class AuthService:
         password: str,
         email: Optional[str] = None,
         guest_id: Optional[str] = None,
+        is_test_account: bool = False,
     ) -> RegistrationResult:
         """
         Register a new user account.
@@ -110,6 +111,7 @@ class AuthService:
             password: Plain text password.
             email: Optional email address.
             guest_id: Guest session ID if converting.
+            is_test_account: Mark this user as a soak-harness test account.
 
         Returns:
             RegistrationResult with user or error.
@@ -151,6 +153,7 @@ class AuthService:
             guest_id=guest_id,
             verification_token=verification_token,
             verification_expires=verification_expires,
+            is_test_account=is_test_account,
         )
 
         if not user: