fix(security): use crypto/rand for alias prefix generation

Replace math/rand.Intn with crypto/rand for generating random alias
prefixes. While aliases are not security tokens, using a CSPRNG ensures
consistent use of cryptographically secure randomness throughout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

addy.go

@@ -3,13 +3,13 @@ package main
 import (
 	"bytes"
 	"crypto/hmac"
+	"crypto/rand"
 	"crypto/sha256"
 	"encoding/base32"
 	"encoding/json"
 	"flag"
 	"fmt"
 	"log"
-	"math/rand"
 	"net/http"
 	"os"
 	"strings"
@@ -230,10 +230,14 @@ func addyAliasAPIDelete(w http.ResponseWriter, r *http.Request) {
 }
 
 func generateRandomString(length int) string {
-	charset := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
+	const charset = "abcdefghijklmnopqrstuvwxyz0123456789"
 	result := make([]byte, length)
-	for i := range result {
-		result[i] = charset[rand.Intn(len(charset))]
+	buf := make([]byte, length)
+	if _, err := rand.Read(buf); err != nil {
+		panic("crypto/rand unavailable: " + err.Error())
+	}
+	for i, b := range buf {
+		result[i] = charset[int(b)%len(charset)]
 	}
 	return string(result)
 }