fix: harden mirror push rollback

[chriswritescode-dev]

Summary

• harden mirror upload swaps with unique backup paths and rollback restoration
• ensure mirror target parents exist before final rename
• include local frontend scroll/test cleanup already present on the branch

Validation

• pnpm --filter backend exec vitest run test/routes/internal/repo-mirror.test.ts
• pnpm --filter backend lint
• pnpm lint
• pnpm typecheck