feat(postcards): add notes field to postcards with edit, detail, and truncated list display

app/templates/postcard_form.html

@@ -33,6 +33,9 @@
         <input type="date" name="receive_time" id="input-receive" value="{{ postcard.receive_time.strftime('%Y-%m-%d') if postcard and postcard.receive_time else '' }}">
     </div>
 
+    <label>备注</label>
+    <textarea name="notes" rows="3" placeholder="可选备注信息">{{ postcard.notes if postcard else '' }}</textarea>
+
     <div class="form-actions">
         <button type="submit" class="btn btn-primary">保存</button>
         <a href="/profiles/{{ profile.id }}/postcards" class="btn">取消</a>