T5 - Dashboard Fixes: active technicians, interactive bars, quick actions, caching

- Fix Active Technicians: query users WHERE role='technician' instead of counting from visit data
- Interactive bar charts: hover tooltips show label+count, click drills down to filtered asset list
- Quick action icons: coin for Assets, Sites, Check-in replaces CSV export buttons
- No visit data: suggestion card with 'Add a Check-in' button
- No manufacturer data: hide section entirely when empty
- 30-second TTL cache on both backend (/api/stats) and frontend (loadDashboard)
- Add dashboard cache module with time-based invalidation

.gitignore

@@ -5,3 +5,5 @@ key.pem
 cert.pem
 __pycache__/
 .venv/
+tests/test_assets_master.db-*
+tests/test_map_api.db-*