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

frontend/src/api/dashboard.ts

@@ -0,0 +1,57 @@
+import request from './request'
+
+export interface DashboardStats {
+  data_sources: number
+  tables: number
+  columns: number
+  labeled: number
+  sensitive: number
+  projects: number
+}
+
+export interface LevelDistItem {
+  name: string
+  code: string
+  color: string
+  count: number
+}
+
+export interface CategoryDistItem {
+  name: string
+  count: number
+}
+
+export interface SourceDistItem {
+  source: string
+  count: number
+}
+
+export interface ProjectProgressItem {
+  id: number
+  name: string
+  status: string
+  progress: number
+  planned_end: string | null
+}
+
+export interface HeatmapItem {
+  source_name: string
+  level_code: string
+  count: number
+}
+
+export interface DashboardDistribution {
+  level_distribution: LevelDistItem[]
+  category_distribution: CategoryDistItem[]
+  source_distribution: SourceDistItem[]
+  project_progress: ProjectProgressItem[]
+  heatmap: HeatmapItem[]
+}
+
+export function getDashboardStats() {
+  return request.get('/dashboard/stats')
+}
+
+export function getDashboardDistribution() {
+  return request.get('/dashboard/distribution')
+}