refactor: restructure core module into domain-driven packages with 2672 insertions and 2220 deletions

Splits monolithic core module into bounded contexts: identity, billing, and notification domains. Moves entity definitions, repository interfaces, and service implementations into separate packages. Updates import paths across 25 files to align with new package structure. Removes cross-domain coupling by extracting shared value objects into a common package.

.backup.1.rhttp.c

@@ -0,0 +1,72 @@
+#include "rmalloc.h"
+#include "rhttp.h"
+#include "rtest.h"
+#include <pthread.h>
+
+int request_handler(rhttp_request_t *r) {
+    rhttp_send_drain(r->c,
+                     "HTTP/1.1 200 OK\r\n"
+                     "Content-Length: 3\r\n"
+                     "Connection: close\r\n\r\n"
+                     "Ok!",
+                     0);
+    close(r->c);
+    return 1;
+}
+
+rhttp_request_handler_t handler = request_handler;
+
+void *rhttp_serve_thread(void *port_arg) {
+    int port = *(int *)port_arg;
+    rhttp_serve(rhttp_opt_host, port, 1024, rhttp_opt_request_logging, rhttp_opt_debug, handler, NULL);
+    return NULL;
+}
+
+int main(int argc, char *argv[]) {
+    bool do_test = true;
+    int port = 9876;
+
+    for (int i = 1; i < argc; i++) {
+        if (!strcmp(argv[i], "--serve")) {
+            printf("rhttp serve mode\n");
+            printf("Handlers available:\n");
+            printf(" - rhttp_root (/)\n");
+            printf(" - rhttp_counter (/counter*)\n");
+            printf(" - rhttp_404 (/*)\n");
+            do_test = false;
+        }
+        if (!strcmp(argv[i], "--quiet")) {
+            rhttp_opt_info = false;
+            rhttp_opt_warn = false;
+            rhttp_opt_request_logging = false;
+            rhttp_opt_debug = false;
+            printf("Quiet mode enabled\n");
+        }
+        if (atoi(argv[i])) {
+            port = atoi(argv[i]);
+        }
+    }
+    if (do_test) {
+        rtest_banner("rhttp");
+    } else {
+        printf("Serving on %s:%d\n", rhttp_opt_host, port);
+        handler = rhttp_default_request_handler;
+    }
+
+    pthread_t st;
+    pthread_create(&st, 0, rhttp_serve_thread, (void *)&port);
+
+    char *response = rhttp_client_get("127.0.0.1", port, "/");
+
+    if (do_test) {
+        rassert(!strcmp(response, "Ok!"));
+        pthread_cancel(st);
+        // cleanup
+    } else {
+        pthread_join(st, NULL);
+    }
+    // rhttp_main(argc, argv);
+    if (do_test)
+        return rtest_end("");
+    return 0;
+}