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-server/src/domain/keys.rs

@@ -16,6 +16,7 @@ pub struct KeyService {
 }
 
 impl KeyService {
+    /// Upload an MLS KeyPackage for the given identity. Returns a SHA-256 fingerprint.
     pub fn upload_key_package(
         &self,
         req: UploadKeyPackageReq,
@@ -40,6 +41,7 @@ impl KeyService {
         Ok(UploadKeyPackageResp { fingerprint })
     }
 
+    /// Fetch the stored MLS KeyPackage for an identity. Returns empty if none exists.
     pub fn fetch_key_package(
         &self,
         req: FetchKeyPackageReq,
@@ -53,6 +55,7 @@ impl KeyService {
         })
     }
 
+    /// Upload a hybrid (ML-KEM-768) public key for post-quantum key exchange.
     pub fn upload_hybrid_key(
         &self,
         req: UploadHybridKeyReq,
@@ -70,6 +73,7 @@ impl KeyService {
         Ok(())
     }
 
+    /// Fetch the hybrid public key for a single identity. Returns empty if none exists.
     pub fn fetch_hybrid_key(
         &self,
         req: FetchHybridKeyReq,
@@ -82,6 +86,7 @@ impl KeyService {
         Ok(FetchHybridKeyResp { hybrid_public_key })
     }
 
+    /// Batch-fetch hybrid public keys for multiple identities. Missing keys return empty.
     pub fn fetch_hybrid_keys(
         &self,
         req: FetchHybridKeysReq,