Implement Tagging for job applications

Implement tag management for job applications. Allow users to create tags, attach them to applications, remove them, and filter applications by one or more tags. Ensure tags are per user and not global.

Look at the existing job application models and database schema to understand how to add a many-to-many relationship for tags. Start by creating a Tag model and a join table for applications and tags, ensuring user-scoping. Implement CRUD endpoints for tags and update the application filtering logic. Check for existing API routes and tests to see patterns.

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