Creates two real DB users with conversations, messages, agent configs,
preferences, and insights, then verifies that every repository query path
(ConversationsRepo, AgentsRepo, PreferencesRepo, InsightsRepo) returns only
data belonging to the requesting user.
28 tests cover: own-data access, cross-user read isolation, cross-user write
prevention, system-agent visibility for all users, and vector-search scoping.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>