feat(backend): implement task creation and admin web

docs/design/admin-task-create.html

@@ -210,6 +210,12 @@
               <input id="title" name="title" maxlength="120" placeholder="例如：演示用桌面收纳盒">
               <p id="title-error" class="error"></p>
             </div>
+            <div class="span-2">
+              <label class="required" for="sku">SKU</label>
+              <input id="sku" name="sku" maxlength="120" placeholder="例如：浅灰色 · 中号">
+              <p id="sku-error" class="error"></p>
+              <p class="help">SKU 是不可覆盖的原始采购要求。</p>
+            </div>
             <div class="span-2">
               <label for="description">商品描述</label>
               <textarea id="description" name="description" placeholder="填写规格、颜色、材质等判断条件"></textarea>
@@ -221,7 +227,7 @@
               <p id="quantity-error" class="error"></p>
             </div>
             <div>
-              <label for="budget">最高总预算（元）</label>
+              <label for="budget">最高商品总预算（元）</label>
               <input id="budget" name="budget" type="number" min="0.01" step="0.01" inputmode="decimal" placeholder="不填写则不设上限">
               <p id="budget-error" class="error"></p>
             </div>
@@ -284,7 +290,7 @@
     });
 
     function clearFeedback() {
-      ['title', 'quantity', 'budget', 'image'].forEach((name) => {
+      ['title', 'sku', 'quantity', 'budget', 'image'].forEach((name) => {
         document.querySelector(`#${name}-error`).textContent = '';
       });
       notice.style.display = 'none';
@@ -303,18 +309,23 @@
       clearFeedback();
       let firstInvalid = null;
       const title = document.querySelector('#title');
+      const sku = document.querySelector('#sku');
       const quantity = document.querySelector('#quantity');
       const budget = document.querySelector('#budget');
       if (!title.value.trim()) {
         document.querySelector('#title-error').textContent = '请输入商品标题。';
         firstInvalid ||= title;
       }
+      if (!sku.value.trim()) {
+        document.querySelector('#sku-error').textContent = '请输入 SKU。';
+        firstInvalid ||= sku;
+      }
       if (!Number.isInteger(Number(quantity.value)) || Number(quantity.value) < 1) {
         document.querySelector('#quantity-error').textContent = '数量必须是大于 0 的整数。';
         firstInvalid ||= quantity;
       }
       if (budget.value && Number(budget.value) <= 0) {
-        document.querySelector('#budget-error').textContent = '预算必须大于 0，或留空。';
+        document.querySelector('#budget-error').textContent = '商品总预算必须大于 0，或留空。';
         firstInvalid ||= budget;
       }
       if (!image.files.length) {