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/events/store/jsonl.py

@@ -51,6 +51,7 @@ class JsonlRunEventStore(RunEventStore):
                         record = json.loads(line)
                         max_seq = max(max_seq, record.get("seq", 0))
                     except json.JSONDecodeError:
+                        logger.debug("Skipping malformed JSONL line in %s", f)
                         continue
         self._seq_counters[thread_id] = max_seq
 
@@ -73,6 +74,7 @@ class JsonlRunEventStore(RunEventStore):
                 try:
                     events.append(json.loads(line))
                 except json.JSONDecodeError:
+                    logger.debug("Skipping malformed JSONL line in %s", f)
                     continue
         events.sort(key=lambda e: e.get("seq", 0))
         return events
@@ -89,6 +91,7 @@ class JsonlRunEventStore(RunEventStore):
             try:
                 events.append(json.loads(line))
             except json.JSONDecodeError:
+                logger.debug("Skipping malformed JSONL line in %s", path)
                 continue
         events.sort(key=lambda e: e.get("seq", 0))
         return events