fix: replace MongoDB LINQ provider with Builders<T>.Filter (#13)

- Remove MongoDB.Driver.Linq dependency from stores
- Replace all LINQ expressions with Builders<T>.Filter API
- Fix ExpressionNotSupportedException with .Equals() in LINQ3
- Use ElemMatch for embedded collection queries
- Use in-memory queries for post-fetch filtering on embedded data

This resolves the MongoDB LINQ3 translation error:
"Expression not supported: x.UserId.Equals(...)"