Remove instance backup and restore feature

[tilyupo]

Summary

• Remove the instance backup and restore API, UI, core transfer logic, tests, documentation, and INSTANCE_TRANSFER_TOKEN configuration.
• Restore affected application files to their state before the transfer feature was added.
• Retain the unrelated benchmark latency threshold changes from the same original commit.

Validation

• pnpm typecheck passed.
• pnpm lint passed with one existing <img> warning in src/components/workspace-switcher.tsx.
• git diff HEAD^ -- . ':!bench/scripts/spec.ts' was empty after the revert, confirming only the benchmark change remains relative to the pre-feature commit.
• pnpm test could not pass because PostgreSQL at 127.0.0.1:5433 is unavailable here (299 passed; 553 failed with ECONNREFUSED).