fix: Add JoinColumn decorators to InviteCode entity relations

Fixed TypeORM query error where column 'creatorId' was not found. The database uses 'created_by' but TypeORM was defaulting to 'creatorId' for the ManyToOne relation. Added @JoinColumn decorators to specify correct column names for all relations.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

maternal-app/maternal-app-backend/src/modules/invite-codes/invite-codes.entity.ts

@@ -1,4 +1,4 @@
-import { Entity, Column, PrimaryGeneratedColumn, CreateDateColumn, UpdateDateColumn, ManyToOne, OneToMany } from 'typeorm';
+import { Entity, Column, PrimaryGeneratedColumn, CreateDateColumn, UpdateDateColumn, ManyToOne, OneToMany, JoinColumn } from 'typeorm';
 import { User } from '../../database/entities/user.entity';
 
 @Entity('invite_codes')
@@ -13,6 +13,7 @@ export class InviteCode {
   createdBy: string;
 
   @ManyToOne(() => User, { nullable: true })
+  @JoinColumn({ name: 'created_by' })
   creator?: User;
 
   @Column({ default: 0 })
@@ -52,12 +53,14 @@ export class InviteCodeUse {
   inviteCodeId: string;
 
   @ManyToOne(() => InviteCode, code => code.usages)
+  @JoinColumn({ name: 'invite_code_id' })
   inviteCode: InviteCode;
 
   @Column({ name: 'used_by' })
   usedBy: string;
 
   @ManyToOne(() => User)
+  @JoinColumn({ name: 'used_by' })
   user: User;
 
   @Column({ name: 'user_email' })