Improve test isolation with improved schema reset

[teoljungberg]

Replace database_cleaner with a custom DatabaseReset module that
drops all non-system schemas between tests, ensuring complete isolation.

Changes:

• Add DatabaseReset module that drops all user schemas (not just public)
• Remove no unused database_cleaner gem
• Add ensure block for search_path reset in multi-schema test
• Update load_defaults to 7.2 (minimum supported Rails version)
• Fix acceptance helper to use DatabaseReset

This tries to fix the intermittent test failures caused by leftover
schemas like test_schema and other persisting between test runs.