feat: add Workday ATS integration

[bisgeario]

Adds Workday as a new ATS integration following the same patterns established by the existing Greenhouse and Lever integrations.

What was missing: Workday was not supported as an ATS provider despite being a widely-used HR platform.

What changed:

• Added workday to the Prisma TP_ID enum in schema.prisma
• Added Workday config variables (WORKDAY_CLIENT_ID, WORKDAY_CLIENT_SECRET, WORKDAY_TOKEN_URL, WORKDAY_API_BASE_URL) to config.ts and .env.example
• Extended ATS_TP_ID type and added Workday to DEFAULT_SCOPE and mapIntegrationIdToIntegrationName in constants/common.ts
• Created WorkdayAuthHandler in routes/v1/ats/authHandlers/workday.ts implementing OAuth2 token exchange (modeled after the Lever handler)
• Wired up case TP_ID.workday in the ATS auth router
• Added Workday cases to all four ATS service files (candidate, job, department, offer) — GET/list/create/update endpoints map to Workday's /api/staffing/v6/ REST API; delete endpoints return "not supported" consistent with Workday API constraints
• Added Workday to the proxy service for generic pass-through requests
• Added Workday field mappings across all four ATS object types in prisma/fields.ts
• Added case TP_ID.workday to disunifyAtsObject and the preprocess map
• Added Workday to the client-side appsInfo metadata

Workday uses tenant-specific base URLs, so app_config.org_url is used per-connection with a fallback to the WORKDAY_API_BASE_URL env var. Pagination uses offset-based cursors derived from the total field in responses.

Testing

Ran tsc -p tsconfig.json --noEmit in packages/backend — only a pre-existing deprecation warning about baseUrl (unrelated to this change), no new type errors. The generated .prisma/client already includes workday in the TP_ID enum, confirming the schema change is consistent with the expected client types.

Closes #372
/claim #372

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