mig(skills): support skill version promotion

[danielkov]

Summary

• add an optional promotion timestamp for restoring an existing immutable skill version without duplicating canonical content
• index effective version ordering for current-version resolution
• index project and skill scoped feedback pagination

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Summary by cubic

Add skill version promotion via optional promoted_at on skill_versions to restore an existing version without duplicating canonical content. Create concurrent indexes for effective ordering by COALESCE(promoted_at, created_at) and for project+skill feedback pagination (project_id, skill_id, created_at DESC, id DESC), and thread promoted_at through DB/repo models and query responses so current-following distributions observe restored content (DNO-574).

^{Written for commit 19b12c2. Summary will update on new commits.}

[linear-code]

DNO-574

[changeset-bot]

⚠️ No Changeset found

Latest commit: 19b12c2

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[cubic-dev-ai]

All reported issues were addressed across 3 files

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>

[github-actions]

atlas migrate lint on server/migrations

Status	Step	Result
	1 new migration file detected	20260728192302_dno_574_skill_version_promotion.sql
	ERD and visual diff generated	View Visualization
	No issues found	View Report
Read the full linting report on Atlas Cloud

[github-actions]

atlas migrate lint on server/clickhouse/migrations

Status	Step	Result
	No migration files detected
	ERD and visual diff generated	View Visualization
	No issues found	View Report
Read the full linting report on Atlas Cloud