fix(pricing): 手动调价支持负数降价对冲

之前手动调价只允许非负整数，无法用负数对冲降价。

前端 (admin-web settings/page.tsx):
- 移除 input min="0" 限制，允许输入负数
- 验证改为：只校验 isNaN 和总价不低于 0（15831 + amount >= 0）
- 文案："加价金额" → "调价金额"，placeholder 改为"正数涨价，负数降价"
- 实时预览条件从 amount >= 0 改为总价 >= 0
- 提示文案更新为"正数涨价，负数降价对冲"

后端 (admin-service tree-pricing.service.ts):
- 移除 newSupplement < 0 的硬性拒绝
- 改为校验 BASE_PRICE(15831) + newSupplement >= 0，防止总价为负

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/admin-service/src/pricing/tree-pricing.service.ts

@@ -79,8 +79,10 @@ export class TreePricingService {
     reason: string,
     operatorId: string,
   ): Promise<TreePricingConfigResponse> {
-    if (newSupplement < 0) {
-      throw new Error('加价金额不能为负数');
+    // 允许负数（降价对冲），但总价不能低于 0
+    const BASE_PRICE = 15831;
+    if (BASE_PRICE + newSupplement < 0) {
+      throw new Error(`调价金额不能低于 -${BASE_PRICE}，否则总价为负`);
     }
 
     const config = await this.getOrCreateConfig();

frontend/admin-web/src/app/(dashboard)/settings/page.tsx

@@ -321,21 +321,26 @@ export default function SettingsPage() {
   // 手动修改加价金额
   const handleSupplementUpdate = useCallback(async () => {
     const amount = parseInt(newSupplement, 10);
-    if (isNaN(amount) || amount < 0) {
-      alert('请输入有效的加价金额（非负整数）');
+    if (isNaN(amount)) {
+      alert('请输入有效的调价金额（整数）');
+      return;
+    }
+    if (15831 + amount < 0) {
+      alert(`调价金额不能低于 -15831，否则总价为负`);
       return;
     }
     if (!supplementReason.trim()) {
       alert('请填写变更原因');
       return;
     }
-    if (!confirm(`确认将加价金额修改为 ${amount} USDT？\n正式认种总价将变为 ${15831 + amount} USDT/棵\n预种总价将变为 ${Math.floor((15831 + amount) / 5)} USDT/份`)) return;
+    const totalPrice = 15831 + amount;
+    if (!confirm(`确认将调价金额修改为 ${amount} USDT？\n正式认种总价将变为 ${totalPrice.toLocaleString()} USDT/棵\n预种总价将变为 ${Math.floor(totalPrice / 5).toLocaleString()} USDT/份`)) return;
     setPricingSaving(true);
     try {
       await treePricingService.updateSupplement(amount, supplementReason.trim());
       setSupplementReason('');
       await loadPricingConfig();
-      alert('加价金额修改成功');
+      alert('调价金额修改成功');
     } catch (error) {
       console.error('Failed to update supplement:', error);
       alert('修改失败，请重试');
@@ -1180,15 +1185,14 @@ export default function SettingsPage() {
           {/* 手动调价 */}
           <div className={styles.settings__quotaCard}>
             <div className={styles.settings__quotaHeader}>
-              <span className={styles.settings__quotaTitle}>手动调价（总部运营成本压力涨价）</span>
+              <span className={styles.settings__quotaTitle}>手动调价（总部运营成本压力调价）</span>
             </div>
             <div className={styles.settings__quotaInputRow}>
-              <span className={styles.settings__quotaText}>加价金额</span>
+              <span className={styles.settings__quotaText}>调价金额</span>
               <input
                 type="number"
-                min="0"
                 className={cn(styles.settings__input, styles['settings__input--medium'])}
-                placeholder="加价金额"
+                placeholder="正数涨价，负数降价"
                 value={newSupplement}
                 onChange={(e) => setNewSupplement(e.target.value)}
               />
@@ -1214,8 +1218,8 @@ export default function SettingsPage() {
               </button>
             </div>
             <span className={styles.settings__hint}>
-              加价部分全额归总部 (S0000000001)，不影响现有 10 类分配金额。
-              {newSupplement && !isNaN(parseInt(newSupplement, 10)) && parseInt(newSupplement, 10) >= 0 && (
+              调价部分全额归总部 (S0000000001)，不影响现有 10 类分配金额。正数涨价，负数降价对冲。
+              {newSupplement && !isNaN(parseInt(newSupplement, 10)) && (15831 + parseInt(newSupplement, 10)) >= 0 && (
                 <> 修改后: 正式认种 {(15831 + parseInt(newSupplement, 10)).toLocaleString()} USDT/棵，预种 {Math.floor((15831 + parseInt(newSupplement, 10)) / 5).toLocaleString()} USDT/份</>
               )}
             </span>