Feat: add submit api stale alert and dashboard pannel

[tripura-repalle]

Closes: #67

Summary

Adds staleness monitoring for the Submit API (alert + dashboard panel) and fixes a bug that prevented any Demeter alert rules from being provisioned.

Submit API staleness

The Submit API is a stateless HTTP proxy that forwards transaction submissions to a cardano-node; it exposes no chain tip of its own. If the node it forwards to has a frozen chain tip, submissions are relayed against a stale tip with no visibility today. This PR detects that condition via the backing node's block-height progress.

• Alert — grafana/alerts/demeter/submit-api.json
  • Group Submit API, folder Demeter, rule "Submit API backing node is stale", for: 10m, receiver #infra-alerts.
  • Condition: changes(cardano_node_metrics_blockNum_int{namespace="ext-nodes-m1", pod=~"node-.*"}[15m]) < 1 — fires when the backing node is up but its block height has not advanced for at least 15 minutes.
• Dashboard — new panel "Node Block Advancement (changes / 15m)" (id 20) added to the SubmitAPI dashboard (grafana/dashboards/shared/submit-api.json), red@0 / green@1, filtered by $network.

This is complementary to the existing node.json alert: that one detects a node whose series disappears, whereas this detects a node that is up but frozen.

Fix: alert folder was never created

Instantiating the grafana_alerts module surfaced a pre-existing bug: the module only created its folder when the configured folder_uid was empty, but config.yaml always supplies a concrete UID (e.g. de9p56uby56v4a for Demeter). As a result the folder was never created, and every rule group failed on apply with:

[PUT /v1/provisioning/folder/{FolderUID}/rule-groups/{Group}][400] invalid alert rule: folder does not exist

The module now always manages its folder with the config.yaml title/uid (mirroring the grafana_dashboard module), guarded by a count so a missing/empty alert directory (e.g. grafana/alerts/blinklabs) stays a no-op and never creates a stray folder. Rule groups reference the managed folder's uid, and the now-unused folders variable is removed.

Validation

terraform fmt clean and terraform validate passes. A plan on this branch is clean:

Plan: 8 to add, 1 to change, 0 to destroy.

• adds the Demeter alert folder + all rule groups (cluster, fabric, kupo, kupo-dolos, node, submit-api)
• updates the SubmitAPI dashboard in-place (the new panel)
• Blink Labs alerts remain a no-op (empty directory)