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

app/chats/router.py

@@ -8,6 +8,7 @@ from app.chats.schemas import (
     ChatMemberAddRequest,
     ChatMemberRead,
     ChatMemberRoleUpdateRequest,
+    ChatPinMessageRequest,
     ChatRead,
     ChatTitleUpdateRequest,
 )
@@ -17,6 +18,7 @@ from app.chats.service import (
     get_chat_for_user,
     get_chats_for_user,
     leave_chat_for_user,
+    pin_chat_message_for_user,
     remove_chat_member_for_user,
     update_chat_member_role_for_user,
     update_chat_title_for_user,
@@ -58,6 +60,7 @@ async def get_chat(
         id=chat.id,
         type=chat.type,
         title=chat.title,
+        pinned_message_id=chat.pinned_message_id,
         created_at=chat.created_at,
         members=members,
     )
@@ -127,3 +130,13 @@ async def leave_chat(
     current_user: User = Depends(get_current_user),
 ) -> None:
     await leave_chat_for_user(db, chat_id=chat_id, user_id=current_user.id)
+
+
+@router.post("/{chat_id}/pin", response_model=ChatRead)
+async def pin_chat_message(
+    chat_id: int,
+    payload: ChatPinMessageRequest,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user),
+) -> ChatRead:
+    return await pin_chat_message_for_user(db, chat_id=chat_id, user_id=current_user.id, payload=payload)