feat(P4B.T2): WhiteboardPayload + shared whiteboards CRUD wrappers + tests

Adds WhiteboardPayload wire-format variant to MessagePayload/ParsedMessagePayload,
creates the whiteboards.ts CRUD wrapper (createWhiteboard, listWhiteboardStrokes,
insertWhiteboardStroke, clearWhiteboardStrokes), writes 5 unit tests (38/38 pass),
re-exports from chat/index.ts, and registers conversation_whiteboards +
whiteboard_strokes in packages/db-types so typecheck passes cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/shared/src/chat/attachments.ts

@@ -76,7 +76,17 @@ export interface PollPayload {
   options: PollOption[];
 }
 
-export type MessagePayload = TextMessagePayload | CallEventPayload | PollPayload;
+export interface WhiteboardPayload {
+  v: 1;
+  type: 'whiteboard';
+  whiteboard_id: string;
+}
+
+export type MessagePayload =
+  | TextMessagePayload
+  | CallEventPayload
+  | PollPayload
+  | WhiteboardPayload;
 
 export type ParsedMessagePayload =
   | {
@@ -95,6 +105,10 @@ export type ParsedMessagePayload =
       kind: 'poll';
       question: string;
       options: PollOption[];
+    }
+  | {
+      kind: 'whiteboard';
+      whiteboardId: string;
     };
 
 export function serializeMessagePayload(payload: MessagePayload): string {
@@ -151,6 +165,13 @@ export function parseMessagePayload(raw: string | null): ParsedMessagePayload {
           options,
         };
       }
+      if (obj.type === 'whiteboard') {
+        const p = obj as Partial<WhiteboardPayload>;
+        const id = typeof p.whiteboard_id === 'string' && p.whiteboard_id.length > 0
+          ? p.whiteboard_id
+          : '';
+        return { kind: 'whiteboard', whiteboardId: id };
+      }
       const t = obj as TextMessagePayload;
       return {
         kind: 'text',