Convenience endpoint for the schedule-creation UI.

Returns everything the "New schedule" form needs in a single call so the modal doesn't have to juggle project slug / multiple endpoints:

• senders — redacted ProjectEmailSender rows for the dashboard's owning project
• webhooks — redacted ProjectSlackWebhook rows for the project
• macros — the allow-list of rolling-window macro names
• timezones— a curated short-list of common IANA zones so users don't have to type one in blindly (they can still hand-edit the field if they need something exotic)
• quota — current schedule count + project plan limit + a boolean flag the UI uses to grey out the "New" button when the limit is reached

Anyone with require_manage=True can call this (creating a schedule is gated the same way, so there's no point showing the form to members who can't submit it).