journal API: + asset_code a error_msg — diagnostika selhaných zápisů bez SSH

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/routers/sites.py

@@ -414,6 +414,8 @@ class ModbusJournalCommandRow(BaseModel):
     status: str
     attempt_count: int
     created_at: str
+    asset_code: str | None = None
+    error_msg: str | None = None
 
 
 class ModbusJournalListResponse(BaseModel):