feat(core): clear saved chat and add message deletion scopes

backend:

- add message_hidden table for per-user message hiding

- support DELETE /messages/{id}?for_all=true|false

- implement delete-for-me vs delete-for-all logic by chat type/permissions

- add POST /chats/{chat_id}/clear and route saved chat deletion to clear

web:

- saved messages action changed from delete to clear

- message context menu now supports delete modal: for me / for everyone

- add local store helpers removeMessage/clearChatMessages

- include realtime stability improvements and app error boundary

web/src/store/chatStore.ts

@@ -22,6 +22,8 @@ interface ChatState {
   confirmMessageByClientId: (chatId: number, clientMessageId: string, message: Message) => void;
   removeOptimisticMessage: (chatId: number, clientMessageId: string) => void;
   setMessageDeliveryStatus: (chatId: number, messageId: number, status: DeliveryStatus) => void;
+  removeMessage: (chatId: number, messageId: number) => void;
+  clearChatMessages: (chatId: number) => void;
   setTypingUsers: (chatId: number, userIds: number[]) => void;
   setReplyToMessage: (chatId: number, message: Message | null) => void;
   updateChatPinnedMessage: (chatId: number, pinnedMessageId: number | null) => void;
@@ -137,6 +139,22 @@ export const useChatStore = create<ChatState>((set, get) => ({
       messagesByChat: { ...state.messagesByChat, [chatId]: next }
     }));
   },
+  removeMessage: (chatId, messageId) => {
+    const old = get().messagesByChat[chatId] ?? [];
+    set((state) => ({
+      messagesByChat: {
+        ...state.messagesByChat,
+        [chatId]: old.filter((m) => m.id !== messageId)
+      }
+    }));
+  },
+  clearChatMessages: (chatId) =>
+    set((state) => ({
+      messagesByChat: {
+        ...state.messagesByChat,
+        [chatId]: []
+      }
+    })),
   setTypingUsers: (chatId, userIds) =>
     set((state) => ({ typingByChat: { ...state.typingByChat, [chatId]: userIds } })),
   setReplyToMessage: (chatId, message) =>