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/db-types/src/index.ts

@@ -477,6 +477,67 @@ export type Database = {
           },
         ]
       }
+      conversation_whiteboards: {
+        Row: {
+          id: string
+          conversation_id: string
+          owner_user_id: string
+          created_at: string
+        }
+        Insert: {
+          id?: string
+          conversation_id: string
+          owner_user_id: string
+          created_at?: string
+        }
+        Update: {
+          id?: string
+          conversation_id?: string
+          owner_user_id?: string
+          created_at?: string
+        }
+        Relationships: [
+          {
+            foreignKeyName: "conversation_whiteboards_conversation_id_fkey"
+            columns: ["conversation_id"]
+            isOneToOne: false
+            referencedRelation: "conversations"
+            referencedColumns: ["id"]
+          },
+        ]
+      }
+      whiteboard_strokes: {
+        Row: {
+          id: string
+          whiteboard_id: string
+          author_user_id: string
+          stroke_json: Json
+          created_at: string
+        }
+        Insert: {
+          id?: string
+          whiteboard_id: string
+          author_user_id: string
+          stroke_json: Json
+          created_at?: string
+        }
+        Update: {
+          id?: string
+          whiteboard_id?: string
+          author_user_id?: string
+          stroke_json?: Json
+          created_at?: string
+        }
+        Relationships: [
+          {
+            foreignKeyName: "whiteboard_strokes_whiteboard_id_fkey"
+            columns: ["whiteboard_id"]
+            isOneToOne: false
+            referencedRelation: "conversation_whiteboards"
+            referencedColumns: ["id"]
+          },
+        ]
+      }
     }
     Views: {
       [_ in never]: never