@NotNull should also apply to DDL

[jo-elimu]

The JPA schema export at https://github.com/elimu-ai/webapp/blob/main/src/main/resources/META-INF/jpa-schema-export.sql should apply not null when an entity's property is annotated with @NotNull.

https://stackoverflow.com/questions/51258073/notnull-annotation-does-not-translate-to-not-null-constraint-in-schema-generati

Make changes so that @NotNull automatically adds not null to the database layer (DDL + production database).

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Relates to #1286, #1607