Add Zod validation for tenants API

[Eldren-cmd]

Closes #892

Summary

• Adds POST /api/tenants and PATCH /api/tenants/:tenantId.
• Adds strict Zod schemas for tenant create/update requests and route params.
• Returns standardized structured 400 validation envelopes.
• Requires auth for tenant writes.
• Adds structured tenant create/update logging with request and correlation IDs.
• Documents the visible API contract in docs/tenants-api.md.
• Fixes the error handler envelope helper reference so validation errors render correctly.

Testing

• jest src/validators/tenants.test.ts src/routes/tenants.test.ts --runInBand --forceExit --silent
• Coverage: src/routes/tenants.ts 100% lines/statements, src/validators/tenants.ts 100% lines/statements
• Touched-file lint passed
• Filtered typecheck showed no errors in touched files

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[greatest0fallt1me]

Nice one — clean, focused change. Merging. 🚀