Fix path traversal in template/snapshot names and network cleanup leaks

Add SafeName validator (allowlist regex) to reject directory traversal
in user-supplied template and snapshot names. Validated at both API
handlers (400 response) and sandbox manager (defense in depth).

Refactor CreateNetwork with rollback slice so partially created
resources (namespace, veth, routes, iptables rules) are cleaned up
on any error. Refactor RemoveNetwork to collect and return errors
instead of silently ignoring them.

internal/validate/name_test.go

@@ -0,0 +1,41 @@
+package validate
+
+import "testing"
+
+func TestSafeName(t *testing.T) {
+	tests := []struct {
+		name    string
+		input   string
+		wantErr bool
+	}{
+		{"simple", "minimal", false},
+		{"with-dash", "template-abc123", false},
+		{"with-dot", "my-snapshot.v2", false},
+		{"sandbox-id", "sb-12345678", false},
+		{"single-char", "a", false},
+		{"numbers", "123", false},
+		{"max-length", "abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz01", false},
+
+		{"empty", "", true},
+		{"dot-dot", "..", true},
+		{"single-dot", ".", true},
+		{"leading-dot", ".hidden", true},
+		{"slash", "foo/bar", true},
+		{"backslash", "foo\\bar", true},
+		{"traversal", "../etc/passwd", true},
+		{"embedded-traversal", "foo/../bar", true},
+		{"space", "foo bar", true},
+		{"too-long", "abcdefghijklmnopqrstuvwxyz012345678901abcdefghijklmnopqrstuvwxyz01", true},
+		{"absolute", "/etc/passwd", true},
+		{"tilde", "~root", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := SafeName(tt.input)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("SafeName(%q) error = %v, wantErr %v", tt.input, err, tt.wantErr)
+			}
+		})
+	}
+}