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.

Generated with [Devin](https://devin.ai)

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

apps/backend/openapi/bookra.openapi.yaml

@@ -1431,6 +1431,30 @@ paths:
                 $ref: "#/components/schemas/ActivityFeedResponse"
         "404":
           description: Tenant membership not found
+  /v1/activity/read:
+    post:
+      tags: [Activity]
+      operationId: markActivityRead
+      security:
+        - bearerAuth: []
+      summary: Mark all activity events as read
+      description: >-
+        Persists the caller's read cursor — activity events at or before the
+        returned timestamp count as read for this user on subsequent loads.
+      responses:
+        "200":
+          description: Read cursor persisted
+          content:
+            application/json:
+              schema:
+                type: object
+                required: [readAt]
+                properties:
+                  readAt:
+                    type: string
+                    format: date-time
+        "404":
+          description: Tenant membership not found
   /v1/metrics/staff:
     get:
       tags: [Metrics]
@@ -2180,12 +2204,19 @@ components:
           format: date-time
     ActivityFeedResponse:
       type: object
-      required: [events]
+      required: [events, unreadCount]
       properties:
         events:
           type: array
           items:
             $ref: "#/components/schemas/ActivityEvent"
+        unreadCount:
+          type: integer
+          description: Events newer than the caller's persisted read cursor
+        readAt:
+          type: string
+          format: date-time
+          description: Caller's read cursor; absent until the first mark-read
     StaffMetric:
       type: object
       required: [staffId, displayName, bookings, completed, cancelled, utilizationPercent, revenueCents]