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-plugin-api/src/lib.rs

@@ -8,7 +8,7 @@
 //! extern "C" int32_t qpc_plugin_init(HookVTable *vtable);
 //! ```
 //!
-//! The server passes a zeroed [`HookVTable`] to `qpc_plugin_init`.  The plugin
+//! The server passes a zeroed [`HookVTable`] to the init function.  The plugin
 //! fills in whichever function pointers it cares about and returns `0` on
 //! success or a negative error code on failure.  Unused slots remain null and
 //! the server treats them as no-ops.
@@ -27,9 +27,9 @@
 //!
 //! # Return values
 //!
-//! Hooks that can reject an operation return [`HookResult`].  The server maps
-//! `HOOK_CONTINUE` to `HookAction::Continue` and any other value to
-//! `HookAction::Reject` with the reason string from [`HookVTable::error_message`].
+//! Hooks that can reject an operation return an `i32` result code.  The server maps
+//! [`HOOK_CONTINUE`] to allow and any other value to reject, reading the reason
+//! string from [`HookVTable::error_message`].
 
 #![no_std]
 
@@ -105,7 +105,7 @@ pub struct CFetchEvent {
 
 // ── HookVTable ────────────────────────────────────────────────────────────────
 
-/// C-ABI function-pointer table filled by [`qpc_plugin_init`].
+/// C-ABI function-pointer table filled by the plugin's `qpc_plugin_init` export.
 ///
 /// All fields default to null (no-op).  The server only calls a slot when its
 /// pointer is non-null.  The `user_data` field is passed as the first argument