feat: add common employee signup and email API contracts

[Aniruddha-Shriwant]

Description

• Adds OpenAPI contracts for five Common employee, organization-signup, and secondary-email operations.
• Documents OAuth security for the two employee operations and explicit unauthenticated security for the three routes whitelisted by fyle-auth.
• Adds request, response, and runtime error schemas derived from platform-api routes, Marshmallow schemas, actions, fixtures, and current clients.
• Regenerates reference/common.yaml with Redocly CLI 2.19.0.
• Does not add x-internal; these contracts remain public.

Runtime evidence

• Existing YAML fixtures cover approver_allowed, signup/validate, and secondary_emails/verify.
• employees/is_mobile_verified currently has no API YAML fixture.
• orgs/signup has focused core pytest coverage but no API YAML fixture; test additions remain deferred.

Validation

• Redocly 2.19.0 bundle completed successfully.
• Redocly lint reports no findings for the five new operations. The Common root still has five pre-existing security-defined errors and one pre-existing info-license warning.
• git diff --check passed.

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