Add per-metric remediation plans and improve CI pipeline

Per-metric remediation plan scoping (GitLab issue #19):
- Add metric_id column to compliance_item_history table (migration)
- Extend PATCH /items/:hostname/metadata to accept metric_id/metric_ids
  for targeting specific metrics instead of all active items
- Add MetricChipSelector UI in detail panel for choosing which metrics
  to apply resolution_date and remediation_plan changes to
- Display per-metric labels (MetricChip or 'All metrics') on history entries
- Backward compatible: omitting metric_ids preserves hostname-level behavior

CI/CD pipeline improvements:
- Add migration idempotency integration test (runs against real Postgres)
- Add post-deploy smoke tests for compliance and VCL endpoints
- Bump lint --max-warnings from 10 to 25
- Configure varsIgnorePattern for _ prefix convention on unused vars

Closes #19

backend/migrations/run-all.js

@@ -25,6 +25,7 @@ const POSTGRES_MIGRATIONS = [
     'add_flexible_jira_ticket_creation.js',
     'add_multi_item_jira_ticket.js',
     'drop_jira_status_check_constraint.js',
+    'add_compliance_history_metric_id.js',
 ];
 
 async function runAll() {