fix: support plain equality in _where

[LaplaceYoung]

Summary

• Treat primitive values in _where JSON as equality filters.
• Keep the existing operator-object syntax for eq, gt, contains, and related operators.
• Add regression coverage for matchesWhere() and the HTTP _where query path.

Closes #1757

Verification

• pnpm test
• pnpm typecheck
• pnpm lint
• pnpm exec oxfmt --check src/app.test.ts src/matches-where.test.ts src/matches-where.ts
• git diff --check