fix: add privilege migration note for db dump

[7ttp]

Problem

When migrating schemas between projects using supabase db dump
table privileges are not preserved. Tables in the target database inherit ALL privileges from default privileges, causing specific GRANT statements to have no effect.

Solution

Document the privilege migration behavior and provide the SQL command to run before restoring.

Related

• Closes Privileges are not working correctly during migrating between projects due to default privileges being ALL for anon, authenticated #4862

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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📝 Walkthrough

Summary by CodeRabbit

• Documentation
  • Expanded Supabase database restoration documentation with new guidance on privilege migration. The addition explains potential conflicts when default privileges override custom settings during restoration to new projects. Includes detailed pre-restoration procedures and practical SQL examples to help preserve specific database permissions and access controls.

Walkthrough

This pull request adds a "Note on Privilege Migration" section to the Supabase database dump documentation. The new section explains that default privileges assigned to anon and authenticated roles in a new project can override specific privileges from a database dump. It provides a SQL workaround that revokes default privileges before restoring the dump, ensuring that the dumped privileges are correctly applied during migration between Supabase projects.

Assessment against linked issues

Objective	Addressed	Explanation
Document the privilege migration issue when restoring to a new project [#4862]	✅
Provide a workaround to prevent default privileges from overriding dumped privileges [#4862]	✅

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