feat: YDB backend port from ydb-v12.2.1 onto v10.1.5

[kprokopenko]

• Replace external xorm with in-tree pkg/util/xorm (+ core, YDB dialect, sqlite util)
• Wire migrator.YDB, YDBDialect, RecursiveQueries CTE hook, migration YDB() SQL
• Port SQL/store/service changes that exist on v10.1.5 (library elements, search, resource permissions, stats paths, etc.)
• Add YDB Go SDK deps; pin grpc via replace for compatibility
• Makefile: test-go-integration-ydb target
• Docs: docs/sources/setup-grafana/ydb-database.md

Omits v12-only paths (unified storage, etc.) and full serverlock rewrite for 12.x.

Made-with: Cursor

What is this feature?

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Why do we need this feature?

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Who is this feature for?

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Which issue(s) does this PR fix?:

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Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[github-actions]

🚫 [doc-validator] _{reported by reviewdog 🐶}
The first heading 'YDB as Grafana database (experimental, fork)' does not match the title ''.
Decide which of the first heading or title is most applicable and change the other to match, or update them both to a new title.

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. It will be closed in 2 weeks if no further activity occurs. Please feel free to give a status update or ping for review. Thank you for your contributions!

[github-actions]

This pull request has been automatically closed because it has not had any further activity in the last 2 weeks. Thank you for your contributions!