feat: add `backend-trpc` module

[ronak-create]

Summary

Add tRPC as a backend option (or as an add-on layer) for building fully type-safe APIs with no schema or codegen step.

Why

tRPC is the default choice for Next.js + Express stacks where the frontend and backend share a TypeScript monorepo.
It eliminates an entire class of type/contract mismatches between client and server.

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Design consideration

tRPC is unusual: it's not a standalone server — it runs on top of an existing HTTP framework (Express, Hono, Next.js API routes).

This means it likely fits better as an add-on (foundation add trpc) rather than a backend replacement.
Discussion on the right category is welcome.

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Files to generate

• src/trpc/router.ts — root router with example procedure
• src/trpc/context.ts — context factory
• src/trpc/trpc.ts — initTRPC initialisation
• src/trpc/client.ts — typed client (for use in frontend)
• Integration patch into existing:
  • server.ts (Express middleware mount)
  • OR src/app/api/trpc/[trpc]/route.ts (Next.js App Router adapter)

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Implementation hints

• Key dependencies:

  • @trpc/server
  • @trpc/client
  • @trpc/react-query (if TanStack Query is also selected)

• compatibility.requires:

  • at least one of:
    • backend-express
    • backend-hono
    • frontend-nextjs

• When both tRPC and TanStack Query are selected:

  • @trpc/react-query bridge should be wired automatically

• This is a cross-cutting module:

  • onBeforeCompose hook may be needed to inspect selected backend and inject the correct adapter

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Contributor guidance

Refer to CONTRIBUTING.md → "Adding a New Module"

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Acceptance criteria

• foundation add trpc (or create-time selection) wires tRPC into the selected backend
• Generated router and client type-check end-to-end with zero errors
• React client is wired when frontend-nextjs + TanStack Query are also selected
• Snapshot tests for Express + tRPC and Next.js + tRPC combinations