feat: v1→v2 wallet key migration on server startup

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

main.go

@@ -75,6 +75,11 @@ func main() {
 	}
 	chainHandler := chain.NewHandler(chainSvc)
 
+	// Migrate v1 wallets to v2 (HKDF per-wallet keys)
+	if err := chainSvc.MigrateWalletKeys(); err != nil {
+		log.Fatalf("wallet key migration failed: %v", err)
+	}
+
 	userResolver := func(username string) (uint, error) {
 		user, err := authRepo.FindByUsername(username)
 		if err != nil {