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/internal/activity/service_test.go

@@ -0,0 +1,82 @@
+package activity
+
+import (
+	"context"
+	"testing"
+
+	"bookra/apps/backend/internal/db"
+	"bookra/apps/backend/internal/domain"
+)
+
+var demoPrincipal = domain.Principal{Subject: "demo-owner", Role: "owner"}
+
+func TestListCountsUnreadAgainstCursor(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	svc := NewService(repo)
+	ctx := context.Background()
+
+	feed, err := svc.List(ctx, demoPrincipal, 10)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(feed.Events) == 0 {
+		t.Fatal("expected seeded demo events")
+	}
+	if feed.UnreadCount != len(feed.Events) {
+		t.Fatalf("expected all %d events unread without cursor, got %d", len(feed.Events), feed.UnreadCount)
+	}
+	if feed.ReadAt != nil {
+		t.Fatalf("expected nil readAt before first mark, got %v", feed.ReadAt)
+	}
+}
+
+func TestMarkAllReadPersistsCursor(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	svc := NewService(repo)
+	ctx := context.Background()
+
+	readAt, err := svc.MarkAllRead(ctx, demoPrincipal)
+	if err != nil {
+		t.Fatalf("mark all read: %v", err)
+	}
+	if readAt.IsZero() {
+		t.Fatal("expected non-zero readAt")
+	}
+
+	feed, err := svc.List(ctx, demoPrincipal, 10)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if feed.UnreadCount != 0 {
+		t.Fatalf("expected 0 unread after mark, got %d", feed.UnreadCount)
+	}
+	if feed.ReadAt == nil || !feed.ReadAt.Equal(readAt) {
+		t.Fatalf("expected readAt %v, got %v", readAt, feed.ReadAt)
+	}
+
+	// A new event after the cursor is unread again.
+	if err := repo.AppendActivityEvent(ctx, db.ActivityEventParams{
+		TenantID:   "5d6b3551-0a3e-4b86-bdf0-e9df20a47148",
+		ActorKind:  "staff",
+		Action:     "booking.created",
+		EntityType: "booking",
+		Summary:    "New — Booking",
+	}); err != nil {
+		t.Fatalf("append event: %v", err)
+	}
+	feed, err = svc.List(ctx, demoPrincipal, 10)
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if feed.UnreadCount != 1 {
+		t.Fatalf("expected 1 unread after new event, got %d", feed.UnreadCount)
+	}
+}
+
+func TestMarkAllReadRequiresMembership(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	svc := NewService(repo)
+	if _, err := svc.MarkAllRead(context.Background(), domain.Principal{Subject: "nobody"}); err == nil {
+		t.Fatal("expected membership error for unknown subject")
+	}
+}