Ensure username is required for new users

Update database schema to make the username field mandatory for all users.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: a8a78c07-e900-425c-a577-5b4c5894379d
Replit-Commit-Checkpoint-Type: full_checkpoint
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shared/schema.ts

@@ -30,7 +30,7 @@ export const sessions = pgTable(
 // User storage table - supports both Replit Auth and local auth
 export const users = pgTable("users", {
   id: varchar("id").primaryKey().notNull(),
-  username: varchar("username").unique(), // For simple login
+  username: varchar("username").unique().notNull(), // For simple login
   email: varchar("email").unique(),
   name: varchar("name"), // Single name field for compatibility
   firstName: varchar("first_name"),