Post classified submissions to Slack, not just the CMS queue

[ssavutu]

The Approve/Reject interactivity endpoint has been in place for a while, but nothing ever posted the message those buttons live on — the CMS was verifying clicks that could not arrive. This adds the outbound half of the moderation loop.

What's here

• server/internal/slack — a Notifier that renders the Block Kit message (submitter, email, category, run-until date, the message itself, Approve/Reject buttons, and an optional link back to the CMS queue) and posts it to an incoming webhook. The button action_ids and the row id they carry are the contract with internal/handlers/slack.go, so both ends now share exported constants. The handler still accepts the past-tense spellings used by messages already sitting in channel history.
• POST /v1/classifieds now notifies on success. The post runs in a goroutine on a context detached from the request — the response is written immediately after, which would otherwise cancel it mid-flight — and a failure logs rather than 500s. The row is already in the queue, and a reader must not see an error for a moderation channel they do not know exists.
• Reader-supplied text is escaped and blockquoted so a submission cannot forge links or mimic the CMS's own formatting, and is truncated before Slack's 3000-character section limit, which would otherwise reject the whole message rather than just the tail.
• Configuration is two halves, and half is worse than none. A webhook without a signing secret puts buttons in the channel that the callback refuses; a signing secret without a webhook posts nothing to click. Startup now names whichever variable is missing, and the moderation queue only promises Slack approvals when both are present (SlackModerationConfigured). A malformed webhook URL is fatal at boot rather than silently failing on every submission.

Config

SLACK_WEBHOOK_URL (new, required for notifications) and SLACK_CLASSIFIEDS_QUEUE_URL (new, optional link back to the CMS) added to both compose files and both env examples. Running with neither is still supported — submissions simply wait in the CMS queue.

Testing

go build, go vet, and go test ./... are clean; new tests cover webhook URL validation, the button contract, non-200 error surfacing, escaping, and truncation, plus a table test that SlackModerationConfigured requires both halves. Frontend tsc -b and eslint are clean. Not yet exercised against a real Slack workspace — that needs a webhook URL in the Delta env.

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