chore(server): review removal of legacy migration 39 repair

[incognitojam]

Context

Older fork installations could record the fork Composer Drafts migration as upstream migration 39. The current migration runner contains a one-time compatibility repair for that exact legacy ledger entry: it applies the real upstream migration 39 safely and rewrites the upstream ledger to its canonical name.

The first fixed Fork Nightly was published on August 9, 2026:
https://github.com/yngatech/t3code/releases/tag/v0.0.33-nightly.20260809.126

Review date

Review this compatibility repair for removal on or after October 9, 2026, two months after the first fixed nightly.

Removal criteria

• Known fork users have had a reasonable opportunity to update through a fixed nightly or later release.
• Confirm with the small set of known fork users where practical.
• Preserve a documented recovery path for any installation that still has the legacy migration 39 ledger entry.
• Remove only the one-time legacy migration 39 repair and its repair-specific tests/documentation.
• Keep the separate yngatech_sql_migrations ledger and fork migration 1_ComposerDrafts permanently.
• Run the focused fork migration and startup migration tests before removal.

Relevant code

• apps/server/src/persistence/ForkMigrations.ts
• apps/server/src/persistence/ForkMigrations.test.ts
• docs/internals/fork-migrations.md