Enforce supported Node version for backend

[neryluc]

Enforce supported Node version for the backend

Installing the backend on a newer Node release (e.g. Node 26) fails because the native better-sqlite3 dependency has no prebuilt binary and its build breaks against the current V8 API. The result is a long, confusing C++ compile log that doesn't make the real cause obvious.

Rather than bumping dependencies (which would break users on older Node), this PR pins the supported Node range and gives a clear error:

• package.json — add "engines": { "node": ">=20 <25" } to declare the supported range.
• .npmrc — set engine-strict=true so npm install fails fast with an EBADENGINE message naming the required vs. actual version, instead of attempting the build.
• .node-version — pin 24 so nvm/fnm/mise auto-select a compatible version.
• README.md — note the Node version requirement.

Verified: on Node 26 npm install now stops immediately with a clear EBADENGINE error; on Node 24 it installs cleanly with no build errors.

Note: engine-strict makes the version a hard requirement rather than a warning — happy to drop it if you'd prefer engines to stay advisory.

Open for feedbacks! Love the course! <3