Backend: JPA / spring-data / hibernate improvements

Mark entities / columns / transactions:

• readonly (@Transactional(readOnly = true)): reduces memory consumption
• non-nullable as per DB constrains (@Column(nullable = false)) => properties can have non-nullable type
• Make all associations lazy
• others?

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• https://vladmihalcea.com/jpa-persist-and-merge - The redundant save anti-pattern

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Hibernate L2 caches:

• https://www.baeldung.com/hibernate-second-level-cache and https://stackoverflow.com/questions/31585698/spring-boot-jpa2-hibernate-enable-second-level-cache
• https://vladmihalcea.com/how-does-hibernate-query-cache-work/

Some entities that are worth caching:

• Users, user-project associations (#580, #649) - won't change often, but are required for all permission checks
• Projects, organizations - won't change often, but are often required by frontend

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build time enhancement