Separate notice periods for membership upgrades vs downgrades

Memberships currently have one self-serve change notice period that applies to every member-initiated change, and changes can only take effect on a billing date. Clubs want these split so members can upgrade instantly with no notice, while downgrades still require notice (e.g. 30 days).

Proposed:

• Separate "Upgrade notice period" and "Downgrade notice period" settings per membership

• Detect upgrade vs downgrade by comparing the new price to the current one

• Allow upgrades to take effect immediately rather than only on the next billing date

• Downgrades keep current behaviour: scheduled to a billing date at or after the notice threshold