docs: add crate-level documentation and public API doc comments

- Expand crate-level docs for quicprochat-rpc (architecture, wire format,
  module map) and quicprochat-sdk (connection lifecycle, event subscription,
  module descriptions).
- Add /// doc comments to all undocumented pub fn/struct/enum items in
  server domain services (keys, channels, devices, users, account, p2p,
  blobs) and domain types.
- Fix rustdoc broken intra-doc links in plugin-api (HookResult,
  qpc_plugin_init), federation/mod.rs (Store), and client main.rs
  (unescaped brackets).

crates/quicprochat-client/src/main.rs

@@ -505,7 +505,7 @@ enum Command {
         password: Option<String>,
     },
 
-    /// Interactive 1:1 chat: type to send, incoming messages printed as [peer] <msg>. Ctrl+D to exit.
+    /// Interactive 1:1 chat: type to send, incoming messages printed as \[peer\] msg. Ctrl+D to exit.
     /// In a two-person group, peer is chosen automatically; use --peer-key only with 3+ members.
     Chat {
         #[arg(