A release announces itself where the cluster already alerts

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Parent

#1289

What to build

Flagger emits promotion, rollback and gate-held events and nothing listens to them. Uptime alerts already reach Discord through a webhook materialised from Vault. Point Flagger at the same one so release events land beside them.

Acceptance criteria

• Promotion, failure and rollback events reach Discord
• The webhook is read from the existing Vault-backed Secret and never appears in git
• A held promotion gate is distinguishable in the channel from an outright failure

Blocked by

• api and frontend promote as a pair, or not at all #1294 — needs the Canaries