Update code references and tests: Bookmann → Bookhoard

Code changes:
- main.go: Update cache directory path
- sidecar.go: Update file extension (.bookmann.json → .bookhoard.json)
- security.go: Update CORS example URLs
- queue_test.go: Update test database name
- feed_test.go: Update test assertions
- phase1_integration_test.go: Update test email addresses
- TEST_COVERAGE.md: Update project references

Part of project rename to Bookhoard.

cmd/server/tests/TEST_COVERAGE.md

@@ -1,6 +1,6 @@
 # Test Coverage Report
 
-This document provides a comprehensive overview of all test scenarios covering possible failure points in the Bookmann application.
+This document provides a comprehensive overview of all test scenarios covering possible failure points in the Bookhoard application.
 
 ## Test Files