cleanup: remove 2,369 lines of dead code

Removed backup files and unused legacy scanner function.
All code verified as unreferenced.

aggregator-server/internal/api/handlers/setup.go

@@ -425,6 +425,13 @@ func (h *SetupHandler) GenerateSigningKeys(c *gin.Context) {
 	c.Header("Pragma", "no-cache")
 	c.Header("Expires", "0")
 
+	// Load configuration to check for existing key
+	cfg, err := config.Load() // This will load from .env file
+	if err == nil && cfg.SigningPrivateKey != "" {
+		c.JSON(http.StatusConflict, gin.H{"error": "A signing key is already configured for this server."})
+		return
+	}
+
 	// Generate Ed25519 keypair
 	publicKey, privateKey, err := ed25519.GenerateKey(rand.Reader)
 	if err != nil {