feat: create dashboard overview API endpoint

Add /api/dashboard/stats route that provides admin dashboard statistics:
- Total counts (games, benchmarks, users)
- Pending moderation items (reports, suggestions)
- 30-day activity metrics (recent benchmarks, recent games)
- Top 10 contributors by entry count
- Sync health breakdown by status
- Games by source distribution
- Playability distribution

Auth: requireContributorOrAdmin guard
Route registered in lib/api/index.ts

lib/api/index.ts

@@ -21,3 +21,4 @@ export { playabilityRoutes } from "./playability"
 export { steamReviewRoutes } from "./steam-reviews"
 export { communitySuggestionRoutes } from "./community-suggestions"
 export { savedFilterRoutes } from "./saved-filters"
+export { dashboardRoutes } from "./dashboard"