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

frontend/package.json

@@ -28,7 +28,10 @@
     "extends": [
       "react-app",
       "react-app/jest"
-    ]
+    ],
+    "rules": {
+      "no-unused-vars": ["warn", { "vars": "all", "args": "after-used", "ignoreRestSiblings": true, "varsIgnorePattern": "^_", "argsIgnorePattern": "^_" }]
+    }
   },
   "browserslist": {
     "production": [