fix: preserve and log backtest retry reasons

[hjcud]

Closes #72

What changed

• preserve the handler/orchestrator retry reason in backtest.run_attempts.failure_code while keeping terminal_reason_code=RETRY_RELEASED
• configure worker console logging at startup with BACKTEST_LOG_LEVEL (INFO by default)
• emit retry identifiers and handler exception tracebacks without logging payloads or credentials
• document the retry-attempt evidence semantics and logging control

No database migration is required; the existing failure_code and terminal_reason_code columns are used.

Verification

• 1236 passed, 3 skipped, 168 deselected (non-Docker suite)
• 14 passed (PostgreSQL persistence integration)
• 10 passed (LocalStack SQS worker integration)
• ruff check src tests
• mypy src/backtest_engine tests/conftest.py tests/persistence
• git diff --check