Add test library cleanup by name and documentation

Problem:
- Libraries are universal (not user-owned) and persist in database
- Tests create libraries via API but don't clean them up
- Libraries accumulate between test runs

Solution:
- Delete test libraries (names containing "test") during cleanup
- Uses case-insensitive matching to catch "Test", "TEST", "test", etc.
- Preserves user-created libraries without "test" in name

Changes:
1. cmd/server/tests/test_helpers.go:
   - Added library cleanup in setupTestServer() after user cleanup
   - Lists all libraries and deletes those with "test" in name
   - Includes warning comment about naming convention

2. docs/contributing/development.md:
   - Added "Test Library Naming Convention" section
   - Documents that "test" in library names triggers deletion
   - Recommends alternative names for persistent test libraries

Note: Users should NOT use "test" in library names if they want to keep them.

docs/contributing/development.md

@@ -279,6 +279,15 @@ Environment variables for testing:
 
 **⚠️ WARNING**: Never enable these in production!
 
+### Test Library Naming Convention
+
+Integration tests automatically clean up libraries with "test" in the name (case-insensitive).
+
+**⚠️ IMPORTANT**: Do not use "test" in library names if you want to keep them!
+- Libraries containing "test" (e.g., "My Test Library", "Test Library 1") will be deleted by test cleanup
+- Use names like "Development Library", "Staging Books", or "Personal" for libraries you want to keep
+- This ensures your manual test data persists between test runs
+
 ## 📝 Code Style Guidelines
 
 ### Go Code