feat: 管理者権限による請求書作成（POST /invoices）APIの定義追加 (#46)

[reiroop]

本PRは、Issue #46 に基づき、管理者向けの請求書作成API（POST /invoices）および管理者請求書作成リクエストスキーマ（PostAdminInvoiceRequest）の定義を追加します。また、レスポンススキーマを PostInvoiceResponse に共通化・統合（リネーム）し、既存の /me/invoices もこれを使用するように修正しています。このプルリクエストは #47（feature/issue-45）の上に重ねた（stacked）状態になっています。

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