feat: add Workable ATS integration

[saquibreja7-hash]

Summary

Closes #551.

Adds Workable as an ATS integration across auth, provider constants, Prisma enum/seed data, service routing, and ATS API operations.

Changes

• Add Workable ATS provider registration and auth handler
• Add Workable service implementation for candidates, jobs, departments, offers, and proxy calls
• Add Prisma enum/migration/seed entries for Workable
• Add transform/preprocess/disunify support for Workable ATS objects
• Add focused Workable service unit tests

Verification

• node .yarn/releases/yarn-3.2.2.cjs install --immutable
• node .yarn/releases/yarn-3.2.2.cjs workspace @revertdotdev/backend prisma generate
• Focused TypeScript check for Workable service/auth/test files
• node .yarn/releases/yarn-3.2.2.cjs workspace @revertdotdev/backend test services/ats/workable.test.ts --runInBand
• git diff --check

Note: full backend production build is blocked locally by missing Fern-generated artifacts that require login; focused checks and tests pass.

/claim #551

[github-actions]

Thank you for following the naming conventions for pull request titles! 🙏

[saquibreja7-hash]

Claiming the linked Algora bounty per the issue instructions.

/claim #551