feat: add info field to articles for shop display

Added optional info text field to articles that can be managed in the backend and is displayed in the shop only when filled. The info appears in both card view and detail modal with an informative style.

- Added info column to items table with migration
- Updated backend edit form with textarea for info text
- Modified API to include info field in bestand response
- Enhanced shop frontend to display info badge when available

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

wawi/templates/edit.html

@@ -29,6 +29,10 @@
           <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 style="grid-column: 1 / -1;">
+          Info-Text für Shop (optional)
+          <textarea name="info" rows="3" placeholder="z.B. Lieferzeit 2-3 Wochen, limitierte Auflage, etc.">{{ item.info if item and item.info else '' }}</textarea>
+        </label>
         <label>
           Soll
           <input type="number" name="soll" min="0" value="{{ item.soll if item else 0 }}" />