feat: add reply/forward/pin message flow across backend and web

- add reply_to/forwarded_from message fields and chat pinned_message field

- add forward and pin APIs plus reply support in message create

- wire web actions: Reply, Fwd, Pin and reply composer state

- fix spam policy bug: allow repeated identical messages, keep rate limiting

web/src/chat/types.ts

@@ -6,6 +6,7 @@ export interface Chat {
   id: number;
   type: ChatType;
   title: string | null;
+  pinned_message_id?: number | null;
   created_at: string;
 }
 
@@ -15,6 +16,8 @@ export interface Message {
   sender_id: number;
   type: MessageType;
   text: string | null;
+  reply_to_message_id?: number | null;
+  forwarded_from_message_id?: number | null;
   created_at: string;
   updated_at: string;
   client_message_id?: string;