feat: add email notifications, order references, stock badges, and sale flags

- Add customer email field and PayPal payment reminder emails with order reference (HEL-YEAR-ID format)
- Display stock availability with color-coded badges (available/low/unavailable)
- Add sale/clearance flag with animated red badge overlay
- Implement automatic fallback placeholder for missing/broken product images
- Add email column to order management view

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

wawi/templates/edit.html

@@ -25,6 +25,10 @@
           Bild hochladen (optional)
           <input type="file" name="bild_file" accept="image/*" />
         </label>
+        <label style="display: flex; align-items: center; gap: 8px; padding-top: 8px;">
+          <input type="checkbox" name="sale" value="1" {% if item and item.sale %}checked{% endif %} style="width: auto; height: 18px;" />
+          <span style="color: var(--text);">Sale / Abverkauf 🔥</span>
+        </label>
         <label>
           Soll
           <input type="number" name="soll" min="0" value="{{ item.soll if item else 0 }}" />