fix: add missing @Id annotation to Problem record

[HunarBhatia]

Fixes #130

Problem

The Problem record was missing the @Id annotation on its id field. Without @Id, Spring Data has no way to identify the primary key field, which can cause silent failures in repository operations like findById() and unexpected query behaviour.

Compare with UserProblem which correctly uses @Id:

// UserProblem.java — correct
public record UserProblem(@Id Integer id, ...)

// Problem.java — was missing @Id
public record Problem(int id, ...)

Fix

Added @Id annotation from org.springframework.data.annotation.Id to the id field of the Problem record, consistent with UserProblem.

File Changed

• problems-service/src/main/java/com/hardik/problemsservice/model/Problem.java

[HunarBhatia]

@hardikxk Could you please add labels like bug, backend, and gssoc:approved with a difficulty level so this counts towards GSSoC '26 points? Thank you!