fix: serialize Viewer config writes

[Memtensor-AI]

Problem

The Viewer /api/config read-modify-write path can race when multiple PATCH requests arrive together. In the disposable MemOS evaluation this surfaced as HTTP 500s from competing config writes, which can invalidate evaluation setup and user configuration.

Change

• serialize Viewer config writes with an in-process write lock
• add a 10-request concurrent PATCH regression test
• preserve existing merge and restart behavior

Validation

• npm test -- --run tests/viewer-config.test.ts — 2/2 passed
• Project Harness Full revision 0 — verificationPassed=true, readyForHandoff=true
• exact HEAD: 3726e91d4ed4dd7670598aaa4adc90659a86d522
• Harness evidence uses the full Ruff + pytest + coverage command with HF_HUB_DISABLE_XET=1 to avoid an execution-host-only Xet TLS retry loop.

Formal release approval remains separate from this Draft PR.