alert-mgmt-11: platform rule update (Drop/Restore)

[sradco]

Alert Management API — Part 11/15: platform rule update

Summary

• UpdatePlatformAlertRule: label changes applied via AlertRelabelConfig CRDs — computes effective labels by merging original, existing ARC overrides, and new values
• DropPlatformAlertRule / RestorePlatformAlertRule: disable/re-enable platform alerts by adding/removing drop relabel configs in the ARC
• GetRuleById: resolves a single relabeled rule by alert rule ID
• ARC upsert logic: creates or updates AlertRelabelConfig resources with stamp + label-change + drop configs, deletes ARC when no changes remain
• Platform label validation: filters protected labels, enforces allowed changes
• AlertingRule label update path for labels that can be persisted directly on the CRD
• Preserved drop handling: maintains existing drops for other rules when updating a shared ARC
• Unit tests for platform update, drop/restore, label filtering, and ARC lifecycle

Dependencies

This PR is part of a stacked series. Please review in order.

1. alert-mgmt-01: k8s foundation + health stub skeleton
   2–10. Pending — alert queries, relabel/health, management foundation, endpoints, create, delete, classification
2. → This PR — platform rule update (Drop/Restore)
   12–15. Pending — bulk update, docs/CI/e2e, single-rule API, update user override classifications