fix(#288): Upgrade RSA key size to 4096 bits

Changed modulusLength from 2048 to 4096 in generateKeypair() method
following NIST recommendations for long-term security. Added test to
verify generated keys meet the minimum size requirement.

Security improvement: RSA-4096 provides better protection against
future cryptographic attacks as computational power increases.

Fixes #288

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/federation.service.ts

@@ -57,10 +57,11 @@ export class FederationService {
 
   /**
    * Generate a new RSA key pair for instance signing
+   * Uses RSA-4096 for future-proof security (NIST recommendation)
    */
   generateKeypair(): KeyPair {
     const { publicKey, privateKey } = generateKeyPairSync("rsa", {
-      modulusLength: 2048,
+      modulusLength: 4096,
       publicKeyEncoding: {
         type: "spki",
         format: "pem",