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_persistence_scaffold.py

@@ -225,6 +225,8 @@ class TestEngineLifecycle:
 
             pytest.skip("asyncpg is installed -- cannot test missing-dep path")
         except ImportError:
-            pass
+            # asyncpg is not installed — this is the expected state for this test.
+            # We proceed to verify that init_engine raises an actionable ImportError.
+            pass  # noqa: S110 — intentionally ignored
         with pytest.raises(ImportError, match="uv sync --extra postgres"):
             await init_engine("postgres", url="postgresql+asyncpg://x:x@localhost/x")