Add Slack notifications for analytics deployment workflows (dbt + Terraform)

[SonaliBedge]

Context & Motivation

• Fixes #MFB-877
• Related PR: None

Analytics deployment workflows (dbt-on-merge and terraform-apply) had no visibility into success or failure after a PR merged. Without notifications, a failed dbt build or Terraform apply would go unnoticed until someone manually checked the Actions tab. This adds Slack notifications to the #mfb-releases channel matching the pattern already in place for benefits-api and benefits-calculator.

Changes Made

• Created .github/actions/analytics-slack-notify/action.yml — a reusable
  composite action that sends a Slack Block Kit message with environment,
  status (success/failure), a direct link to the workflow run, and commit
  info (SHA, author, message)
• Updated dbt-on-merge.yml — added Get commit info and Notify Slack (success/failure) steps; fires on staging only since this workflow never
  targets production
• Updated terraform-apply.yml — added same steps; environment label
  (staging or production) is derived dynamically from
  github.event.inputs.environment to match the job's own environment
  expression

Testing

• dbt models to run: N/A - no dbt model changes
• Terraform plan reviewed: N/A - no Terraform resources changes
• Local Metabase tested via docker-compose: N/A
• Other manual testing steps:
• After merging, trigger dbt Build on Merge or Terraform Apply
  manually via workflow_dispatch and confirm a Slack message appears
  in #mfb-releases with correct status, environment label, commit
  info, and working "View Logs" link
• To verify the failure path: temporarily break a step and confirm
  the ❌ message fires

Deployment

• Production Terraform apply needed: no
• Metabase container rebuild/redeploy needed: no
• dbt production run needed (outside nightly cron): no
• Other post-deployment steps:
  Add SLACK_WEBHOOK_URL as a repository-level secret before
  merging (Settings → Secrets and variables → Actions → Repository
  secrets). It must be repository-level, not environment-scoped,
  because the notify steps run outside the environment context

Notes for Reviewers

The composite action lives at .github/actions/analytics-slack-notify/
following the same local action pattern used in benefits-api

• Two separate notifications fire when a PR touches both dbt/ and
  dashboards/ — this is intentional (each links to its own workflow run)
  rather than a combined notification via workflow_run, which is
  significantly more complex and unreliable

• Get commit info uses if: always() so it captures the SHA/author/message
  even when an earlier step failed, ensuring the failure notification
  includes full commit context

• Known limitations: production notifications currently include commit info
  rather than a release tag/URL — a release-based production deployment
  trigger can be added in a follow-up if needed

• Future considerations: could add a #mfb-data-alerts channel for nightly
  dbt cron failures separately from merge-triggered deployment notifications

[andreaHG]

Nice work here! I'm assuming we'll probably want to remove staging references here since that's not active.