[codex] split backend frontend and api client monorepo

[mosobande]

Summary

• split the project into a standard monorepo with apps/backend, apps/web, and packages/api-client
• extract the generated OpenAPI client into @itukuto/api-client with deterministic sync/drift checking
• replace frontend Biome tooling with oxlint/oxfmt for TS/JS and keep backend formatting on Spotless/ktfmt for Gradle and Maven
• update Gradle, Maven, Docker, GitHub Actions, hooks, docs, and gitignore for the new app/package boundaries

Validation

• ./scripts/verify-template.sh --maven
• pnpm lint:frontend:check
• pnpm test:frontend
• ./gradlew :apps:backend:test :apps:backend:integrationTest :apps:backend:spotlessCheck
• node scripts/itukuto-api-sync.mjs --ci --skip-export
• pre-commit: oxlint, oxfmt, spotless
• pre-push: frontend tests, backend compile, workspace typecheck

Notes

• Local Docker build was not run because the local Docker daemon is unavailable; this PR adds a CI Docker build job to validate the image path remotely.
• The long-term Seam platform vision remains deferred; this PR implements the near-term monorepo seam and deterministic API boundary.