feat(server): propagate is_test_account through User model & store

User dataclass, create_user, and all SELECT lists now round-trip the
new column. Value is always FALSE until Task 4 wires the register
flow to the invite code's marks_as_test flag.

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

server/models/user.py

@@ -45,6 +45,7 @@ class User:
         is_banned: Whether user is banned.
         ban_reason: Reason for ban (if banned).
         force_password_reset: Whether user must reset password on next login.
+        is_test_account: True for accounts created by the soak test harness.
     """
     id: str
     username: str
@@ -66,6 +67,7 @@ class User:
     is_banned: bool = False
     ban_reason: Optional[str] = None
     force_password_reset: bool = False
+    is_test_account: bool = False
 
     def is_admin(self) -> bool:
         """Check if user has admin role."""
@@ -100,6 +102,7 @@ class User:
             "is_banned": self.is_banned,
             "ban_reason": self.ban_reason,
             "force_password_reset": self.force_password_reset,
+            "is_test_account": self.is_test_account,
         }
         if include_sensitive:
             d["password_hash"] = self.password_hash
@@ -146,6 +149,7 @@ class User:
             is_banned=d.get("is_banned", False),
             ban_reason=d.get("ban_reason"),
             force_password_reset=d.get("force_password_reset", False),
+            is_test_account=d.get("is_test_account", False),
         )