feat: make storage tier thresholds configurable in system settings

Add StorageGreenThresholdPercent (default 30%) and StorageRedThresholdPercent
(default 10%) to both SystemSettingsDto and the settings UI.

StorageGuardService now reads thresholds from settings instead of hardcoding.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

frontend/src/types.ts

@@ -421,6 +421,8 @@ export interface SystemSettings {
   enableStorageGuard: boolean;
   pauseRecordingWhenFreeSpaceBelowMegabytes: number;
   resumeRecordingWhenFreeSpaceAboveMegabytes: number;
+  storageGreenThresholdPercent: number;
+  storageRedThresholdPercent: number;
   enableRetentionCleanup: boolean;
   retentionDays: number;
   retentionDeleteFiles: boolean;

frontend/src/views/SettingsView.vue

@@ -106,6 +106,8 @@ const form = reactive<SettingsFormModel>({
   enableStorageGuard: true,
   pauseRecordingWhenFreeSpaceBelowMegabytes: 1024,
   resumeRecordingWhenFreeSpaceAboveMegabytes: 4096,
+  storageGreenThresholdPercent: 30,
+  storageRedThresholdPercent: 10,
   enableRetentionCleanup: false,
   retentionDays: 30,
   retentionDeleteFiles: false,
@@ -1097,10 +1099,37 @@ watch(
               </el-form-item>
             </el-col>
           </el-row>
+          <el-row :gutter="16" style="margin-top: 16px">
+            <el-col :span="12">
+              <el-form-item label="绿色水位线：剩余空间高于 (%)">
+                <el-input-number
+                  v-model="form.storageGreenThresholdPercent"
+                  :min="5"
+                  :max="90"
+                  :step="5"
+                  :disabled="!form.enableStorageGuard"
+                />
+                <div class="field-hint">高于此比例时正常录制，低于时拒绝新录制</div>
+              </el-form-item>
+            </el-col>
+            <el-col :span="12">
+              <el-form-item label="红色水位线：剩余空间低于 (%)">
+                <el-input-number
+                  v-model="form.storageRedThresholdPercent"
+                  :min="1"
+                  :max="85"
+                  :step="5"
+                  :disabled="!form.enableStorageGuard"
+                />
+                <div class="field-hint">低于此比例时暂停所有录制和转码，仅保留上传</div>
+              </el-form-item>
+            </el-col>
+          </el-row>
         </el-form>
 
         <div class="helper-panel">
-          恢复阈值建议高于暂停阈值，避免磁盘空间在临界值附近反复抖动。MP4 转码会额外占用中间 TS 文件空间。
+          恢复阈值建议高于暂停阈值，避免磁盘空间在临界值附近反复抖动。MP4 转码会额外占用中间 TS 文件空间。<br/>
+          绿色/红色水位线控制三级存储保护：<b>绿色</b>(正常录制) → <b>黄色</b>(拒绝新录制，现有继续转码上传) → <b>红色</b>(暂停所有录制转码，仅上传清盘)。
         </div>
       </el-card>