GET /api/health/detailed now returns a session_pipeline service entry.
Heuristic:
max(mtime of /data/user_sessions/**/*.jsonl) <=
max(processed_at in session_extraction_state) + grace_seconds
grace_seconds = 2 × verification-detector cadence (default 30 min;
configurable via SCHEDULER_VERIFICATION_DETECTOR_INTERVAL).
When the assert fails, status='warning' (never 'error') with an
actionable detail pointing at the verification-detector scheduler job.
A warning bubbles up to the existing overall='degraded' aggregation —
operators querying /api/health/detailed (or /agnes diagnose system)
get a clear breadcrumb instead of a silently-broken pipeline.
Cold-start case (no session files, or files newer than the grace
window with empty state table) is handled explicitly to avoid noise
on a fresh deploy.
Tests: tests/test_health_session_pipeline.py.
