Send Deployment Notifications to Slack on Successful Deploy

[vjuliaife]

Description:
The TariffShield team has no automated signal when a production or staging deployment succeeds or fails, requiring developers to manually check Render and Vercel dashboards. Adding Slack notifications to the deploy workflows closes this visibility gap and ensures that the team is immediately aware of every deployment outcome, including the deployed git SHA and a direct link to the run.

Acceptance Criteria:

• Create a Slack incoming webhook URL (SLACK_DEPLOY_WEBHOOK_URL) stored as a GitHub Actions secret, connected to a #tariffshield-deploys Slack channel
• The API deploy workflow (.github/workflows/deploy-api.yml) posts a Slack message on success containing: service name, environment (production or staging), deployed git SHA (7 chars), deployer GitHub username, and a link to the GitHub Actions run
• The Vercel preview deploy workflow posts a Slack message on success containing the preview URL so the team can click straight to the deployed preview
• Both workflows post a distinct failure notification with @here mention and a link to the failed run if the deploy or health check step fails
• Slack notifications use the slackapi/slack-github-action action with a structured Block Kit payload (not a plain text webhook) for readability — a template JSON payload is stored in .github/slack/deploy-success.json and .github/slack/deploy-failure.json
• Notifications include a colour-coded attachment: green sidebar for success, red for failure, yellow for in-progress (posted at deploy start and updated on completion using the Slack update API if the action supports it)
• Document the Slack webhook setup steps in docs/deployment.md under "Notifications"

Relevant Files:

• .github/workflows/deploy-api.yml — API deploy workflow (add Slack steps)
• .github/workflows/preview-deploy.yml — Vercel preview workflow (add Slack steps)
• .github/slack/deploy-success.json — Block Kit success message template
• .github/slack/deploy-failure.json — Block Kit failure message template
• docs/deployment.md — deployment runbook (add Notifications section)

[Mistersmile4585]

@Mistersmile4585 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintenaner, I'm a Full Stack Software Engineer with experience building scalable web and mobile applications. I enjoy solving problems, learning quickly, and contributing to meaningful projects.

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