[Docs] Replace boilerplate frontend/README.md and add missing backend/ and contracts/ READMEs

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

frontend/README.md is the unmodified create-next-app boilerplate with nothing FlowFi-specific, and there is no backend/README.md or contracts/README.md at all. New contributors entering a package have no orientation for that package's scripts, env, or layout.

Acceptance criteria

• Replace frontend/README.md with FlowFi-specific content (purpose, scripts, NEXT_PUBLIC_* env, src/ layout)
• Add backend/README.md (scripts, Prisma, env, /v1 API, SSE)
• Add contracts/README.md (stream_contract layout, build/test, wasm target)

Files to touch

• frontend/README.md
• backend/README.md
• contracts/README.md

Out of scope

• Root README.md