Platform notification system (product-wide): email + UI notices #99

Context

Several products need member-facing notifications but cloud-api has no generic notification infrastructure (the auth email sender is purpose-built; no reusable template/helper exists). The workspace-slot lifecycle currently logs structured warnings only — members get no email when their workspace stops at period end, enters the deletion grace window, or is about to be deleted.

What

A small platform-level notification system covering all products (workspaces/slots, memberships, spaces, future ones):

• A reusable email sender in cloud-api (template + recipient + product tag), built on the existing Resend client, with suppression-list respect.
• Product-neutral event hooks so features request notifications without owning delivery: e.g. workspace.stops_at_period_end, workspace.grace_deletion_pending, workspace.deleted_after_grace.
• UI inbox/banners as a later slice (start with email only).

First consumers

1. Workspace slot lifecycle: stop-at-period-end warning, grace-pending warning, final deletion notice (the free-workspace churn lifecycle needs the same three).
2. Membership dunning already has its own emails — migrate it onto the shared sender only if cheap.

Out of scope

• Real-time channels (push/Discord) — later.
• Per-product template ownership — each feature supplies copy.