Fix missing Mongoose ref options on schema fields

[trillium]

Summary

Fixes #2165 — Adds missing ref options to Mongoose schema fields that were defined as plain String or Number, which breaks .populate().

Files Changed

1. backend/models/projectTeamMember.model.js — usersId and projectId converted to ObjectId with refs to User and Project
2. backend/models/checkIn.model.js — eventId and userId converted to ObjectId with refs to RecurringEvent and User
3. backend/models/event.model.js — projectId converted to ObjectId with ref to Project
4. backend/models/recurringEvent.model.js — projectId converted to ObjectId with ref to Project
5. backend/models/user.model.js — currentRole field type corrected
6. backend/models/project.model.js — projectStatus and googleDriveId schema options corrected

What This Does

This is a schema-only change — no API endpoints or UI components are modified. It adds proper { type: Schema.Types.ObjectId, ref: "ModelName" } declarations where plain Strings were previously used as foreign keys.

Why

Without ref options, Mongoose .populate() calls silently return null instead of resolved documents. This blocks features that depend on population, including:

• As a manager, I want a Toggle Feature so that I can Enable Manager Access #2149 — Manager toggle feature (requires populating team member references)
• Relates to ProjectTeamMember schema is the right join table but is bypassed by redundant fields on User and Project #2155

Testing

• Existing tests pass (schema shape is backward-compatible since MongoDB stores ObjectIds as strings when inserted as strings)
• .populate() calls that previously returned null will now correctly resolve referenced documents