feat: add dashboard, file preview metadata, and bandwidth statistics

Dashboard:
- Add DashboardDto, DashboardService with SQL-level aggregate queries
- Add GET /api/dashboard endpoint with real-time system status
- Add repository aggregate methods (CountByAvailability, SumDuration, etc.)
- Add DashboardView.vue as new landing page with KPI cards, storage status, recent sessions, top rooms
- Update router to make dashboard the new / route, add nav item in sidebar

File Preview:
- Add IVideoMetadataService + FfmpegVideoMetadataService for video metadata extraction
- Extend MediaBrowserItemDto with Metadata and ThumbnailUrl fields
- Add includeMetadata param to media browser API, add thumbnail endpoint
- Add SessionPlaylistDto and GET /api/record-sessions/{id}/playlist for continuous playback

Bandwidth Statistics:
- Add -progress pipe:1 to live recording ffmpeg args for bitrate output
- Parse total_size/bitrate/speed from ffmpeg progress lines during recording
- Write bandwidth samples as SystemLogEntry (Category=Bandwidth) every 30s
- Add BandwidthStatisticsService, BandwidthController with session timeline + daily summary
- Add bandwidth TypeScript types

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

frontend/src/router/index.ts

@@ -3,6 +3,7 @@ import { useAuthStore } from "@/stores/auth";
 
 const LoginView = () => import("@/views/LoginView.vue");
 const MainLayout = () => import("@/components/layout/MainLayout.vue");
+const DashboardView = () => import("@/views/DashboardView.vue");
 const LiveRoomsView = () => import("@/views/LiveRoomsView.vue");
 const RecordTasksView = () => import("@/views/RecordTasksView.vue");
 const TranscodeTasksView = () => import("@/views/TranscodeTasksView.vue");
@@ -29,7 +30,8 @@ const router = createRouter({
       children: [
         {
           path: "",
-          redirect: "/live-rooms"
+          name: "dashboard",
+          component: DashboardView
         },
         {
           path: "live-rooms",