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/schemas.py

@@ -11,6 +11,7 @@ class ChatRead(BaseModel):
     id: int
     type: ChatType
     title: str | None = None
+    pinned_message_id: int | None = None
     created_at: datetime
 
 
@@ -43,3 +44,7 @@ class ChatMemberRoleUpdateRequest(BaseModel):
 
 class ChatTitleUpdateRequest(BaseModel):
     title: str = Field(min_length=1, max_length=255)
+
+
+class ChatPinMessageRequest(BaseModel):
+    message_id: int | None = None