fix(monitoring): alert when a Velero schedule has NEVER backed up, or is paused

VeleroNoRecentBackup could not fire for the failure mode that actually happened.
`(time() - velero_backup_last_successful_timestamp{schedule!=""}) > 24*3600` is the
right SHAPE — per-schedule, un-aggregated — but Velero only creates that series once
a backup for the schedule SUCCEEDS, and `time() - <absent series>` yields no series.
So it detects "backed up once, then stopped" and is structurally INERT on "never
backed up". Prod ran with `velero-daily` (the schedule carrying VOLUME DATA) paused
and never-successful: no volume backups at all, nothing alerted, and it was masked
because `velero-daily-resources-only` completed cleanly every 03:00, so every other
backup signal read green while only Kubernetes objects were protected.

A PromQL-only fix is impossible, verified rather than assumed: velero v1.18.1 exports
35 velero_* metric names and every one carrying a `schedule` label is created by a
backup ATTEMPT, not by the Schedule object. A throwaway `paused: true` Schedule on
prod produced ZERO velero_* series, and nothing exposes `spec.paused`. The fact is
not in the TSDB, so the Schedule CRs are exported via kube-state-metrics'
custom-resource-state collector — the path this repo already uses for
kube_node_labels — as kube_velero_schedule_{created,paused,protects_volume_data,info}.
Proven by running KSM v2.20.0 with this exact config against prod's API: the CRD
list/watch rule the chart adds is MANDATORY, not cosmetic — without it KSM emits the
CRS metrics as an empty set, silently.

Three rules, all keyed on PROPERTIES rather than schedule names:
  * VeleroScheduleNeverBackedUp (critical) — a Schedule older than 36h with no
    successful backup ever. The 36h grace comes from creationTimestamp so a newly
    added schedule is not paged before its first run is even due.
  * VeleroSchedulePaused (warning) — the direct root-cause signal; a paused backup
    schedule must never be silently indefinite.
  * VeleroNoVolumeDataBackup (critical) — THE ANTI-MASKING RULE. Fires unless some
    schedule with spec.template.defaultVolumesToFsBackup=true succeeded in 36h, so a
    healthy resources-only schedule can never make the platform look protected.
    Reading the property that DEFINES volume protection (not a name list) matches
    platform/velero/volume-policies.yaml and survives renames. `or vector(0)` is
    load-bearing: `count(<empty>) == 0` returns no series and would be inert —
    absence of the exporter is deliberately the alarm.

Adds the repo's first Prometheus rule-unit-test harness (platform/monitoring/tests/,
wired as the `prom-rules` gate stage). It flattens the COMMITTED PrometheusRule CRs —
never a copy — and pins every rule in the group both ways: quiet on today's healthy
prod state, firing on the reconstructed bad state, with `for:` durations asserted.
Two mutation checks confirm the tests are not vacuous: dropping `or vector(0)` and
breaking the `unless on (schedule)` join each turn the suite red. This is the tool
that makes both of this repo's recurring alert bugs — permanently-firing and
structurally inert — provable offline, which neither yamllint, Kyverno, nor a healthy
cluster can do.

Proof of the fix is by construction; the live metrics cannot show it any more, because
`velero-daily` was unpaused on 2026-09-12 and has since completed 2492/2492 items.
The throwaway probe Schedule and the probe KSM deployment were deleted and the
deletion confirmed; `velero-daily` remains unpaused.

Co-Authored-By: Claude Opus 5 <[email protected]>

ci/Makefile

@@ -12,7 +12,7 @@ CI_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
 
 .DEFAULT_GOAL := help
 
-.PHONY: help spec-immutability lint schema l0 render-golden l1 conformance gate all touched versions tools test
+.PHONY: help spec-immutability lint schema l0 render-golden l1 prom-rules conformance gate all touched versions tools test
 
 help: ## Show this help
 	@awk 'BEGIN{FS=":.*##"} /^[a-zA-Z0-9_-]+:.*##/{printf "  \033[36m%-14s\033[0m %s\n",$$1,$$2}' $(MAKEFILE_LIST)
@@ -32,6 +32,9 @@ render-golden: ## render==golden: canonical render reproduces committed apps/ (n
 l1: ## L1 rendering checks (no cluster; live now)
 	@"$(CI_DIR)l1.sh"
 
+prom-rules: ## promtool rule-unit tests for platform/monitoring alerts (no cluster; live now)
+	@"$(CI_DIR)prom-rules.sh"
+
 conformance: ## L2/L3 in-cluster Chainsaw Job (SKIPS w/o CLUSTER_AVAILABLE=true)
 	@"$(CI_DIR)conformance.sh"