feat(notifications): persistent read cursor + delivery history

notification_cursors table stores one read-cursor per user per tenant;
GET /v1/activity returns unreadCount + readAt, POST /v1/activity/read
persists the cursor. Reminder dispatches now append reminder.sent /
reminder.failed system events so delivery history reaches the feed.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/migrations/00017_notification_cursors.sql

@@ -0,0 +1,15 @@
+-- +goose Up
+-- Persistent read state for the notification feed: one cursor per
+-- dashboard user per tenant. Events with created_at <= read_at count as
+-- read; everything newer is unread. Per-event flags were rejected — the
+-- feed is an audit list, not a per-item inbox.
+CREATE TABLE notification_cursors (
+  tenant_id    uuid NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+  user_subject text NOT NULL,
+  read_at      timestamptz NOT NULL,
+  updated_at   timestamptz NOT NULL DEFAULT now(),
+  PRIMARY KEY (tenant_id, user_subject)
+);
+
+-- +goose Down
+DROP TABLE IF EXISTS notification_cursors;