fix: add missing th_sortable macro for sortable table headers

The subscription templates were importing th_sortable but the macro
didn't exist. Added the macro to tables.html with:
- Clickable header with sort indicator arrows
- Alpine.js integration for sorting state
- Visual feedback for current sort column and direction

Also removed unused empty_state import from subscription-tiers.html.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/subscription-tiers.html

@@ -3,7 +3,7 @@
 {% extends "admin/base.html" %}
 {% from 'shared/macros/alerts.html' import alert_dynamic, error_state %}
 {% from 'shared/macros/headers.html' import page_header_refresh %}
-{% from 'shared/macros/tables.html' import table_wrapper, table_header_custom, th_sortable, empty_state %}
+{% from 'shared/macros/tables.html' import table_wrapper, table_header_custom, th_sortable %}
 {% from 'shared/macros/modals.html' import modal_confirm %}
 
 {% block title %}Subscription Tiers{% endblock %}