refactor(coordinator-api,blockchain-explorer): add response caching and fix timestamp handling

- Add cached decorator to admin stats, job status, payment status, and marketplace stats endpoints
- Configure cache TTLs using get_cache_config for different endpoint types (1min job_list, 30s user_balance, marketplace_stats)
- Import cache_management router and include it in main app with /v1 prefix
- Fix blockchain-explorer formatTimestamp to handle both ISO string and Unix numeric timestamps with type

pytest.ini

@@ -12,8 +12,8 @@ markers =
     integration: Integration tests (may require external services)
     slow: Slow running tests
 
-# Test paths to run
-testpaths = tests/cli apps/coordinator-api/tests/test_billing.py
+# Test paths to run - restored to full coverage
+testpaths = tests
 
 # Additional options for local testing
 addopts =