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/routers/auth.py

@@ -245,11 +245,23 @@ async def register(
             )
 
     # --- Invite code validation ---
+    is_test_account = False
     if has_invite:
         if not _admin_service:
             raise HTTPException(status_code=503, detail="Admin service not initialized")
         if not await _admin_service.validate_invite_code(request_body.invite_code):
             raise HTTPException(status_code=400, detail="Invalid or expired invite code")
+        # Check if this invite flags new accounts as test accounts
+        invite_details = await _admin_service.get_invite_code_details(request_body.invite_code)
+        if invite_details and invite_details.get("marks_as_test"):
+            is_test_account = True
+            logger.info(
+                "test_seed_account_registering",
+                extra={
+                    "username": request_body.username,
+                    "invite_code": request_body.invite_code,
+                },
+            )
     else:
         # No invite code — check if open signups are allowed
         if config.INVITE_ONLY and config.DAILY_OPEN_SIGNUPS == 0:
@@ -277,6 +289,7 @@ async def register(
         username=request_body.username,
         password=request_body.password,
         email=request_body.email,
+        is_test_account=is_test_account,
     )
 
     if not result.success: