feat: dashboard & report APIs with real DB stats

Backend:
- Add /dashboard/stats API (data_sources, tables, columns, labeled, sensitive, projects)
- Add /dashboard/distribution API (level/cat/source distribution, project progress, heatmap)
- Add /reports/stats API (total/auto/manual/reviewed counts + level distribution)
- Fix report download: add template relationship to ClassificationProject
- All stats computed from real DB queries

Frontend:
- Dashboard.vue: replace all hardcoded data with API-driven computed charts
- Report.vue: replace all hardcoded data with API-driven charts
- Add dashboard.ts and report.ts API clients

backend/app/api/v1/__init__.py

@@ -1,6 +1,6 @@
 from fastapi import APIRouter
 
-from app.api.v1 import auth, user, datasource, metadata, classification, project, task, report
+from app.api.v1 import auth, user, datasource, metadata, classification, project, task, report, dashboard
 
 api_router = APIRouter()
 api_router.include_router(auth.router, prefix="/auth", tags=["认证"])
@@ -11,3 +11,4 @@ api_router.include_router(classification.router, prefix="/classifications", tags
 api_router.include_router(project.router, prefix="/projects", tags=["项目管理"])
 api_router.include_router(task.router, prefix="/tasks", tags=["任务管理"])
 api_router.include_router(report.router, prefix="/reports", tags=["报告管理"])
+api_router.include_router(dashboard.router, prefix="/dashboard", tags=["仪表盘"])