Slack Operations Notifications

Synced with starter commit 7d452a6.

Create a Slack Incoming Webhook and set SLACK_WEBHOOK_URL. SDK construction stays in src/integrations/slack.ts; business code queues typed slack_event or slack_error jobs instead of calling Slack from routes.

Use deterministic dedupe keys for noisy events, include request/order/job identifiers for correlation, and omit credentials, tokens, raw request bodies, download URLs, and unnecessary personal data. A missing webhook should be an intentional environment choice, not a hidden production failure.

Delivery retries through the same durable runner as email and cleanup work. Keep /api/cron/jobs authenticated and scheduled, monitor failed jobs, and test duplicate enqueue, Slack timeout/429/5xx, redaction, and recovery. Slack is an alerting channel; the database and structured logs remain the system of record.