Add Invisible SDK Node server example

[JW-Rami]

What this adds:

• Adds a Node.js backend/server example for @invisible-labs/sdk.
• Covers attested sessions, private transfer/refund routes, LP lifecycle entrypoints, storage, stats, events, and coordinator-pending surfaces.
• Keeps live mutations disabled by default behind INVISIBLE_ENABLE_MUTATIONS.

Why it matters:

• Replaces the old worker framing with an idiomatic backend/server example.

How to test:

• npm run typecheck
• npm run build
• npm test
• npm audit --audit-level=moderate