build: enforce and document @yieldvault/api-schemas build order

[CathyZaks]

Summary

Enforce and document the monorepo build order ensuring @yieldvault/api-schemas is built before running backend TypeScript compilation (tsc), development, or type-checking.

Problem

backend/src/middleware/validate.ts imports @yieldvault/api-schemas, which emits type declarations to packages/api-schemas/dist/index.d.ts. In local development environments after a fresh clone, packages/api-schemas has not been built yet, causing tsc --noEmit and backend builds to fail with Cannot find module '@yieldvault/api-schemas'. Furthermore, the backend package.json had duplicate and conflicting build script definitions, and documentation lacked instructions on building shared packages before backend startup.

Solution

1. Deduplicate build scripts in backend/package.json and configure prebuild, predev, and typecheck scripts to invoke build:schemas (npm --prefix ../packages/api-schemas run build) before running TypeScript or starting dev.
2. Add build:schemas, root build, typecheck, and update dev:backend and dev:frontend in the root package.json to ensure schemas are built before running workspace dev servers.
3. Document the required build order in CONTRIBUTING.md, docs/LOCAL_DEV_BOOTSTRAP.md, docs/CONTRIBUTOR_ONBOARDING_CHECKLIST.md, and backend/docs/REQUEST_VALIDATION.md.

Changes

• package.json: Added build:schemas, root build, typecheck, and ensured dev:backend/dev:frontend trigger build:schemas first.
• backend/package.json: Removed duplicated build script entries, added build:schemas, wired predev and prebuild to compile schemas before building/running, and added a typecheck script.
• CONTRIBUTING.md: Documented the requirement to run npm run build:schemas after installing dependencies and clarified the build order dependency.
• docs/LOCAL_DEV_BOOTSTRAP.md: Added the shared schema build step to the bootstrap installation flow with an explanatory note on the build order requirement.
• docs/CONTRIBUTOR_ONBOARDING_CHECKLIST.md: Added npm run build:schemas to Section 2 and referenced the automatic build enforcement in Section 5.
• backend/docs/REQUEST_VALIDATION.md: Added an explicit note explaining the @yieldvault/api-schemas build dependency for backend type validation.

Testing

• Ran npm run build:schemas from backend and confirmed successful compilation of @yieldvault/api-schemas.
• Ran npx vitest run scripts/validate-workspace-test-scripts.test.ts and confirmed all 6 tests pass.

Closes #1371

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