Wire Sentry to real-time notification (Discord/PagerDuty), not the email-only default

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Summary

Sentry's only alert rule for this project is its own shipped default ("Send a notification for high priority issues"), and its only action is email to issue owners, falling through to active members. All 28 currently-open issues are scored high-priority by Sentry, and all 28 are unassigned — so notification has been going to a fallback nobody watches in real time. Wire a real-time action instead.

Requirements

1. Configure Sentry's native Discord integration (org-level, via Sentry's Integrations page) and add "Send a Discord notification" as an action on the existing (or a new) issue alert rule — ideally the same Discord channel/webhook family Alertmanager already posts to, so an operator has one place to watch, not two.
2. Once PagerDuty exists (see the sibling Alertmanager sub-issue under this epic), also configure Sentry's native PagerDuty integration and route severity: critical-equivalent issues there. Sentry has its own priority/severity concept independent of Alertmanager's — map it deliberately, don't assume they line up 1:1.
3. Decide and document: should every new high-priority issue notify, or only ones crossing some volume/recurrence threshold? The current default (any high-priority issue) is what's already configured and has produced 28 open issues with no differentiation between a 1-event one-off and a 4,000-event, 12-day-ongoing incident — consider whether Sentry's alert-rule filters (event frequency, user count) should be used to separate "notify immediately" from "visible on next triage pass."

Deliverables

• Discord action wired and verified (a live test issue actually posts to the channel).
• PagerDuty action wired once the receiver exists (may land in a follow-up commit once the Alertmanager sub-issue's PagerDuty setup is done, since the two can share the same PagerDuty service).
• A documented decision on alert-rule thresholds/filters, not just "notify on everything."

Part of #5007.

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JSO-3625

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PR #5116 shipped a complementary app-level PagerDuty notifier for ops anomalies (closing #4937), but its own description explicitly leaves this issue's ask — wiring Sentry's native Discord/PagerDuty alert-rule integrations — as a separate, still-open follow-up. Leaving open.

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Attempted to configure this directly via Sentry's API rather than just planning around it. Found a live SENTRY_AUTH_TOKEN/SENTRY_ORG on edge-nl-01 and tried it against the org-info, integrations, and project-issues endpoints (both sentry.io and the correct us.sentry.io region per the token's own JWT payload) — all return 403 You do not have permission to perform this action. This token is scoped for something narrower than org/alert-rule administration (most likely just error-event ingestion from the app itself), not for managing integrations or alert rules via the API.

To finish this, I need one of:

• A Sentry auth token scoped with org:integrations + alert:write (Settings → Auth Tokens on sentry.io) — with that, I can wire the Discord action on the existing alert rule and the severity-filter decision (item 3 in this issue) directly via the API.
• Or, if that's not something to hand out, this needs to be done by hand in Sentry's own Integrations page (org-level Discord app install + wiring it into the alert rule action) — I don't have an authenticated browser session against sentry.io in this environment to do that interactively.

Not blocked on anything else — the Discord webhook itself already exists and works (Alertmanager already posts through it, confirmed live in #5009's check), so once Sentry can reach it, this is genuinely just the integration wiring, not new infrastructure.