Prorated upgrades and initial-cycle charges fixed

We reworked how prorated charges are calculated on plan upgrades, channel additions, and addon changes so the amount you see matches what you actually owe for the remainder of the cycle.

• Fixed prorated charges on plan upgrades and the initial charge that runs the first time a card is added.
• Improved addon invoice handling so partial-month addons no longer overlap or double-charge.
• Cancellations are now dated against the plan itself rather than the time the cancellation was requested.
• Capped the billing cycle day at 28 to avoid month-end drift on shorter months.