Add remediation plan and resolution date history tracking

New table compliance_item_history stores an append-only audit trail of
changes to resolution_date and remediation_plan. The current values remain
on compliance_items for fast VCL reporting queries (no double-counting).

Backend:
- Migration: creates compliance_item_history with indexes
- PATCH /items/:hostname/metadata: records old→new in history before updating,
  accepts optional change_reason field (max 500 chars)
- GET /items/:hostname: returns history array (last 10 entries, newest first)
- POST /vcl/bulk-commit: records history for each changed field per hostname

Frontend:
- ComplianceDetailPanel: added change reason input below Save button
- Added Change History section showing field changes with timestamps,
  usernames, old→new values, and reasons
- Re-fetches detail after save to show updated history immediately

Tests updated to match new transaction-based PATCH flow.

backend/migrations/run-all.js

@@ -20,6 +20,7 @@ const POSTGRES_MIGRATIONS = [
     'add_vcl_reporting_columns.js',
     'add_vcl_vertical_metadata.js',
     'add_vcl_multi_vertical.js',
+    'add_compliance_item_history.js',
 ];
 
 async function runAll() {