Add Dependabot config to backend repo for automated dependency updates

[LucasMaupin]

Summary

The `intercom-frontend` repo has a `.github/dependabot.yml` configured for daily npm dependency updates. The `intercom-manager` backend repo has no equivalent, meaning dependency vulnerabilities accumulate undetected until a manual `npm audit` is run.

Several backend devDependencies are already on older major versions (e.g. `@typescript-eslint/eslint-plugin@^5`, `eslint@^8`) which may have accumulated CVEs.

Recommendation

Add `.github/dependabot.yml` to `intercom-manager`:

version: 2
updates:
  - package-ecosystem: "npm"
    directory: "/"
    schedule:
      interval: "daily"
    open-pull-requests-limit: 10
    ignore:
      - dependency-name: "*"
        update-types: ["version-update:semver-major"]

This mirrors the frontend configuration. Major version bumps can be handled manually to avoid unexpected breaking changes.

Context

• Frontend Dependabot config: `intercom-frontend/.github/dependabot.yml`
• Related: Security: Run npm audit fix to address 7 high + 8 moderate dependency vulnerabilities #227 (npm audit high/moderate vulns), Security: High-severity npm vulnerabilities in transitive deps — flatted, lodash, minimatch, picomatch, glob #231 (transitive dep CVEs)

Severity

Low — Process improvement; reduces future security debt.

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Found by automated security audit.