M5: moderation, permissions, channel CRUD, in-app account management

- Server-side moderation & permissions (kick/ban/move/server-mute, channel CRUD).

- Database schema v2: channels, bans; BLAKE2b channel passwords, Argon2id accounts.

- C ABI additions and client-side handling (vc_kick_user, vc_ban_user, vc_set_permission, vc_set_server_mute, vc_move_user, vc_create/edit/delete_channel, vc_create/reset/delete/list_account).

- vccli flags for all M5 operations plus --username/--password auth.

- Four new tests covering permissions, kick/ban/move/mute, admin accounts, channel CRUD.

- Docs: protocol.md envelope updates, security.md channel-password hashing, PROGRESS.md.

server/src/server.cpp

@@ -54,8 +54,8 @@ int Server::run() {
     }
 
     // ── Session registry ─────────────────────────────────────────────────────
-    auto registry = std::make_shared<SessionRegistry>();
-    registry->init_default_channels();
+    auto registry = std::make_shared<SessionRegistry>(db);
+    registry->load_channels();
 
     // ── Worker pool ──────────────────────────────────────────────────────────
     auto workers = std::make_shared<WorkerPool>(3);