docs(dev): document uninstall cleanup checklist in repair.rst - #15706

Summary

repair.rst documented the uninstall repair-step type but not what apps should remove, even though the app guidelines require cleanup on uninstall and correct downgrade handling.

• Add a short uninstall checklist: app config, user preferences, background jobs, appdata files, and optional database tables.
• Note that uninstall also runs on disable, so destructive drops should stay optional / opt-in.
• Note that migrations are not rolled back on downgrade; restore from backup if an older app version must be reinstalled after schema changes.

Closes #15685

Test plan

• Sphinx build of the developer manual succeeds for the repair page
• New Cleaning up on uninstall section lists the checklist items from the issue
• Downgrade note states migrations are not rolled back
• Cross-ref to app-store publishing guidelines resolves

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.