fix(kanban): serialize DB initialization (#24410)

[teknium1]

Salvages #24410 by @psionic73.

serializes kanban_db.connect() schema init with reentrant lock; main has no init lock (only threading is in dispatcher); 64 LOC + regression test, well-scoped

Cherry-picked onto current main with original authorship preserved via rebase merge.

[github-actions]

🔎 Lint report: hermes/hermes-de55f5dd vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 8807 on HEAD, 8807 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 4630 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.