Add a native PagerDuty receiver to Alertmanager for severity:critical alerts

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Summary

alertmanager.local (the live, gitignored deploy config on edge-nl-01) already routes every alert to Discord, and has a commented-out generic webhook_configs block labeled "PagerDuty bridge" ready for when PagerDuty is set up. Alertmanager has a first-class, native pagerduty_configs receiver type — use that instead of a generic webhook bridge.

Requirements

1. Set up a PagerDuty service (Events API v2 integration) for this project, and add a native pagerduty_configs receiver to alertmanager.local using its routing key — not the generic webhook_configs bridge currently stubbed in.
2. Un-comment and wire the severity-split route already sketched in the file's own comments: severity: critical → PagerDuty (short group_wait, tighter repeat_interval), everything else stays on Discord.
3. Verify send_resolved behavior end-to-end (PagerDuty auto-resolves when Alertmanager reports the alert cleared) — this is default-on per Alertmanager but worth confirming against the actual PagerDuty integration type chosen (Events API v2 vs. the legacy "Prometheus" v1 integration type use different fields — pick v2 with routing_key, not v1's service_key, since v2 is current).
4. Keep the backup: .gittensory.yml-style timestamped backup of alertmanager.local before editing, matching this box's existing convention for live config edits.

Deliverables

• pagerduty_configs receiver live and verified (a test alert actually pages).
• Severity-based routing confirmed working (critical → PagerDuty, warning → Discord only).
• Config change on the box, documented (this lives in a gitignored deploy file, not the repo — note where the change actually lives for the next person who looks for it in git and doesn't find it).

Part of #5007.

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JSO-3626

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PR #5116 addressed app-level PagerDuty paging (#4937) but explicitly did not touch Alertmanager's native pagerduty_configs receiver, which this issue requires. Leaving open.

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Verified live state via SSH on edge-nl-01 (/opt/gittensory/alertmanager/alertmanager.local):

• Discord routing is live and working (root route → discord receiver, real webhook configured).
• The PagerDuty piece is genuinely not done — confirmed no PAGERDUTY_* credential anywhere in .env or the alertmanager config; the receiver block is still the commented-out generic-webhook stub. This requires standing up a new PagerDuty service/Events API v2 integration, which is account creation on a third-party vendor — not something I can do on the maintainer's behalf.
• Made one small, safe fix while I was in there: the config's header comment still said "for edge-us-01" (the old, decommissioned box) — corrected to edge-nl-01, with a timestamped backup, and re-validated via amtool check-config (SUCCESS, container healthy, no restart needed since it was comment-only).

To finish this: create a PagerDuty service (Events API v2), then hand me the routing key — the receiver block and severity-based route (severity: critical → PagerDuty, matching the file's own already-sketched comment) are a 2-line uncomment + key paste away, verified via amtool check-config and a real test alert before calling it done.