Fix error when creating publicity participation records

Remove the `created_at` column from the `publicity_participations` table schema in `shared/schema.ts` to resolve a production error related to the column not existing.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 1e4433b5-47ab-464c-b663-fea2e53367dd
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/1e4433b5-47ab-464c-b663-fea2e53367dd/AVGwNHS

shared/schema.ts

@@ -140,7 +140,6 @@ export const publicities = pgTable("publicities", {
 export const publicityParticipations = pgTable("publicity_participations", {
   publicityId: integer("publicity_id").notNull(),
   groupId: integer("group_id").notNull(),
-  createdAt: timestamp("created_at").defaultNow(),
 }, (table) => ({
   pk: primaryKey({ columns: [table.publicityId, table.groupId] })
 }));