Subscription Email Schedule

Build the email schedule for a set of subscriptions.

• subscriptions: rows [name, plan, start_day, duration]. The subscription ends on end_day = start_day + duration. Names are unique.
• changes: rows [name, day, new_plan], in no particular order. A change switches that customer's plan from day onward. Every change satisfies start_day < day < end_day, and a customer has at most one change per day.
• notice_days: how many days before end_day the expiry warning goes out.

Each customer gets these emails, each written as [day, name, text]:

1. On start_day: "Welcome: <plan>" using the initial plan.
2. On each change day: "Changed: <new_plan>".
3. On end_day - notice_days, only if that day is strictly after start_day: "Expiring soon: <plan>", using the plan in effect that day (a change on the same day counts).
4. On end_day: "Expired: <plan>", using the final plan.

Return all emails sorted by day, then by name. If one customer has two emails on the same day, the "Changed" email comes before the "Expiring soon" email.