Add systemd::notify helper (zero-dep sd_notify protocol)

Lifted from fewo-webapp (src/App.cpp). 15-line helper that speaks the
systemd notification protocol directly — no libsystemd link — for
Type=notify services.

Silent no-op when NOTIFY_SOCKET is unset so the same binary runs
unchanged under systemd or as a plain background process.

Supports Linux abstract-namespace sockets.

Unblocks fewo-webapp #451 and its twin extractions for derived projects.

include/oatpp-authkit/systemd/Notify.hpp

@@ -0,0 +1,54 @@
+#ifndef OATPP_AUTHKIT_SYSTEMD_NOTIFY_HPP
+#define OATPP_AUTHKIT_SYSTEMD_NOTIFY_HPP
+
+#include <cstdlib>
+#include <cstring>
+#include <sys/socket.h>
+#include <sys/un.h>
+#include <unistd.h>
+
+namespace oatpp_authkit::systemd {
+
+/**
+ * @brief Protocol-level sd_notify(3) implementation.
+ *
+ * Speaks the systemd notification protocol by writing a datagram to
+ * $NOTIFY_SOCKET — no libsystemd dependency. Used to signal
+ * `Type=notify` services with `READY=1`, `STATUS=...`, `WATCHDOG=1`.
+ *
+ * No-op (silent return) when NOTIFY_SOCKET is unset — the same binary
+ * can run under systemd or as a plain background process without
+ * conditional logic at the call site.
+ *
+ * Supports Linux abstract-namespace sockets (leading '@' in the env
+ * var, mapped to a leading NUL byte in the sockaddr path).
+ *
+ * Example:
+ * @code
+ *   oatpp_authkit::systemd::notify("READY=1\nSTATUS=Accepting connections");
+ *   // … later, from a watchdog thread:
+ *   oatpp_authkit::systemd::notify("WATCHDOG=1");
+ * @endcode
+ */
+inline void notify(const char* state) {
+    const char* sock = std::getenv("NOTIFY_SOCKET");
+    if (!sock || !*sock) return;
+    int fd = ::socket(AF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0);
+    if (fd < 0) return;
+    struct sockaddr_un addr{};
+    addr.sun_family = AF_UNIX;
+    if (sock[0] == '@') {
+        // Linux abstract namespace: leading '@' maps to a NUL byte.
+        addr.sun_path[0] = '\0';
+        std::strncpy(addr.sun_path + 1, sock + 1, sizeof(addr.sun_path) - 2);
+    } else {
+        std::strncpy(addr.sun_path, sock, sizeof(addr.sun_path) - 1);
+    }
+    ::sendto(fd, state, std::strlen(state), MSG_NOSIGNAL,
+             (struct sockaddr*)&addr, sizeof(addr));
+    ::close(fd);
+}
+
+} // namespace oatpp_authkit::systemd
+
+#endif