fix(persistence): address 22 review comments from CodeQL, Copilot, and Code Quality

Bug fixes:
- Sanitize log params to prevent log injection (CodeQL)
- Reset threads_meta.status to idle/error when run completes
- Attach messages only to latest checkpoint in /history response
- Write threads_meta on POST /threads so new threads appear in search

Lint fixes:
- Remove unused imports (journal.py, migrations/env.py, test_converters.py)
- Convert lambda to named function (engine.py, Ruff E731)
- Remove unused logger definitions in repos (Ruff F841)
- Add logging to JSONL decode errors and empty except blocks
- Separate assert side-effects in tests (CodeQL)
- Remove unused local variables in tests (Ruff F841)
- Fix max_trace_content truncation to use byte length, not char length

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

backend/packages/harness/deerflow/runtime/journal.py

@@ -135,7 +135,7 @@ class RunJournal(BaseCallbackHandler):
         self._llm_start_times[str(run_id)] = time.monotonic()
 
     def on_llm_end(self, response: Any, *, run_id: UUID, **kwargs: Any) -> None:
-        from deerflow.runtime.converters import langchain_to_openai_completion, langchain_to_openai_message
+        from deerflow.runtime.converters import langchain_to_openai_completion
 
         try:
             message = response.generations[0][0].message