schema: rm is_dm, rename name→userinfo (json_flat), fix join_msg format

- Remove is_dm column from channels table (dm detection by member count)
- Rename member name column to userinfo — extensible json_flat format
- Add json_flat.c/h library for compact JSON serialization
- Fix join_sig message format: use ch_x25519+ch_ed25519 instead of channel_id string
- Remove name from join_msg body (now in userinfo via json_flat)
- Update chat_core, chat_sync, member_sync, db_manager to new format
- Add topo_strategy.txt — архитектурные заметки по топологии

lib/json_flat.h

@@ -0,0 +1,25 @@
+/**
+ * @file json_flat.h
+ * @brief Парсер плоского JSON без вложенности: {"key":"value",...}
+ *
+ * Только строковые значения. Без malloc, без зависимостей.
+ */
+
+#ifndef JSON_FLAT_H
+#define JSON_FLAT_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef int (*json_flat_cb)(const char* key, const char* value, void* arg);
+
+int json_flat_parse(const char* json, json_flat_cb cb, void* arg);
+int json_flat_get(const char* json, const char* key, char* value_out, size_t value_sz);
+
+#ifdef __cplusplus
+}
+#endif
+#endif