Fix backend test suite failures

[trillium]

Overview

The backend test suite has several pre-existing failures that predate the ESM migration. These need to be resolved so the test suite can serve as a reliable quality gate.

Action Items

• Router tests (auth, events, projects, users) fail because process.env.CUSTOM_REQUEST_HEADER is undefined — tests should set required env vars themselves
• models/event.model.test.js and models/projectTeamMember.model.test.js have updatedDate assertions that fail because Mongoose pre-update hooks aren't firing on findOneAndUpdate and updateMany
• setup-test.js has un-awaited deleteMany() calls causing race conditions between tests
• MongoMemoryServer teardown produces MongoNetworkError: connection closed errors from parallel test execution
• Empty describe blocks cause Vitest to fail (Jest silently skipped them)

Resources/Instructions

• Related: Migrate backend from CommonJS to ES Modules #2133 (ESM migration)
• Backend test runner: Vitest (migrated from Jest as part of Migrate backend from CommonJS to ES Modules #2133)