Implement pipeline statuses and valid transitions

Define a canonical status pipeline for applications and enforce valid state transitions where appropriate. Expose an endpoint that returns the list of supported statuses and use validation to prevent invalid updates.

Look for existing application models and status fields in the codebase. Define a status enum and a transition validation function. Create an endpoint, likely in an API route file, to list statuses. Add validation to update operations to enforce transitions. Check for existing tests related to application updates.

Written by the indexing model from the issue text.