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/whiteboards.ts

@@ -0,0 +1,96 @@
+import type { Json } from '@chat-app/db-types';
+
+import type { AppSupabaseClient } from '../supabase/client';
+
+export interface Whiteboard {
+  id: string;
+  conversationId: string;
+  ownerUserId: string;
+  createdAt: string;
+}
+
+export interface WhiteboardStroke {
+  id: string;
+  whiteboardId: string;
+  authorUserId: string;
+  strokeJson: unknown;
+  createdAt: string;
+}
+
+export async function createWhiteboard(
+  client: AppSupabaseClient,
+  conversationId: string,
+): Promise<Whiteboard> {
+  const { data: session } = await client.auth.getUser();
+  if (!session.user) throw new Error('not authenticated');
+  const { data, error } = await client
+    .from('conversation_whiteboards')
+    .insert({
+      conversation_id: conversationId,
+      owner_user_id: session.user.id,
+    })
+    .select('id, conversation_id, owner_user_id, created_at')
+    .single();
+  if (error) throw error;
+  return {
+    id: data.id,
+    conversationId: data.conversation_id,
+    ownerUserId: data.owner_user_id,
+    createdAt: data.created_at,
+  };
+}
+
+export async function listWhiteboardStrokes(
+  client: AppSupabaseClient,
+  whiteboardId: string,
+): Promise<WhiteboardStroke[]> {
+  const { data, error } = await client
+    .from('whiteboard_strokes')
+    .select('id, whiteboard_id, author_user_id, stroke_json, created_at')
+    .eq('whiteboard_id', whiteboardId)
+    .order('created_at', { ascending: true });
+  if (error) throw error;
+  return data.map((row) => ({
+    id: row.id,
+    whiteboardId: row.whiteboard_id,
+    authorUserId: row.author_user_id,
+    strokeJson: row.stroke_json,
+    createdAt: row.created_at,
+  }));
+}
+
+export async function insertWhiteboardStroke(
+  client: AppSupabaseClient,
+  params: { whiteboardId: string; strokeJson: unknown },
+): Promise<WhiteboardStroke> {
+  const { data: session } = await client.auth.getUser();
+  if (!session.user) throw new Error('not authenticated');
+  const { data, error } = await client
+    .from('whiteboard_strokes')
+    .insert({
+      whiteboard_id: params.whiteboardId,
+      author_user_id: session.user.id,
+      stroke_json: params.strokeJson as unknown as Json,
+    })
+    .select('id, whiteboard_id, author_user_id, stroke_json, created_at')
+    .single();
+  if (error) throw error;
+  return {
+    id: data.id,
+    whiteboardId: data.whiteboard_id,
+    authorUserId: data.author_user_id,
+    strokeJson: data.stroke_json,
+    createdAt: data.created_at,
+  };
+}
+
+export async function clearWhiteboardStrokes(
+  client: AppSupabaseClient,
+  whiteboardId: string,
+): Promise<void> {
+  const { error } = await client
+    .from('whiteboard_strokes')
+    .delete()
+    .eq('whiteboard_id', whiteboardId);
+  if (error) throw error;
+}