API endpoints for the Fix Issues feature

Expose and document public API endpoints for the Fix Issues workflow so warehouses can automate it from their own systems. Requested capabilities: upload photos to a Fix Issue, set the issue reason/category, add comments/notes, and create or update a Fix Issue programmatically. Note: hold until the planned restructuring of how unsellable items appear in Fix Issues lands, then expose endpoints matching the new structure. Requested by Douglas Couto (prepcentertaxfree.com).