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/db/activity.go

@@ -2,6 +2,10 @@ package db
 
 import (
 	"context"
+	"errors"
+	"time"
+
+	"github.com/jackc/pgx/v5"
 )
 
 func (r *PGRepository) AppendActivityEvent(ctx context.Context, params ActivityEventParams) error {
@@ -40,3 +44,28 @@ func (r *PGRepository) ListActivityEvents(ctx context.Context, tenantID string,
 	}
 	return events, rows.Err()
 }
+
+func (r *PGRepository) GetNotificationCursor(ctx context.Context, tenantID string, userSubject string) (*time.Time, error) {
+	var readAt time.Time
+	err := r.pool.QueryRow(ctx, `
+		SELECT read_at FROM notification_cursors
+		WHERE tenant_id = $1 AND user_subject = $2
+	`, tenantID, userSubject).Scan(&readAt)
+	if errors.Is(err, pgx.ErrNoRows) {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return &readAt, nil
+}
+
+func (r *PGRepository) SetNotificationCursor(ctx context.Context, tenantID string, userSubject string, readAt time.Time) error {
+	_, err := r.pool.Exec(ctx, `
+		INSERT INTO notification_cursors (tenant_id, user_subject, read_at)
+		VALUES ($1, $2, $3)
+		ON CONFLICT (tenant_id, user_subject)
+		DO UPDATE SET read_at = EXCLUDED.read_at, updated_at = now()
+	`, tenantID, userSubject, readAt)
+	return err
+}