ESP abstracted behind a common interface

When the ESP SDK is imported and called in six different worker files, route handlers, and service modules, switching providers requires auditing and rewriting every one of those call sites. More practically, a team that needs to rotate to a backup ESP during an outage must make coordinated changes across the codebase under time pressure. ISO-25010:2011 maintainability.modifiability requires that changes to one component do not cascade to unrelated modules — direct SDK spread is the structural cause of that cascade.

Create an EmailProvider interface in lib/email/types.ts and a concrete adapter in lib/email/providers/sendgrid.ts. All other application code calls only the interface:

// lib/email/types.ts
export interface EmailProvider {
  send(message: EmailMessage): Promise<{ messageId: string }>
  healthCheck(): Promise<boolean>
}

The ESP SDK import belongs in at most 2 files: the adapter implementation and its test. Workers, route handlers, and service modules import the interface, not @sendgrid/mail or equivalent.