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/tests/test_feedback.py

@@ -117,14 +117,16 @@ class TestFeedbackRepository:
     async def test_delete(self, tmp_path):
         repo = await _make_feedback_repo(tmp_path)
         created = await repo.create(run_id="r1", thread_id="t1", rating=1)
-        assert await repo.delete(created["feedback_id"]) is True
+        deleted = await repo.delete(created["feedback_id"])
+        assert deleted is True
         assert await repo.get(created["feedback_id"]) is None
         await _cleanup()
 
     @pytest.mark.anyio
     async def test_delete_nonexistent(self, tmp_path):
         repo = await _make_feedback_repo(tmp_path)
-        assert await repo.delete("nonexistent") is False
+        deleted = await repo.delete("nonexistent")
+        assert deleted is False
         await _cleanup()
 
     @pytest.mark.anyio