Restructure compute core by extracting job contracts and runtime logic into separate
modules (`contracts.ts`, `local-runtime.ts`) for improved modularity and clearer
API boundaries. Update exports and imports across worker, server, and local
compute backends to use the new modules. Enhance Next.js config to support
dynamic backend selection via build-time constants and aliasing, enabling
seamless switching between local and worker compute modes. Adjust TypeScript
paths and Dockerfile entrypoint to align with the new structure.
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