Add custom search queries to repositories

[NikosPoly]

Currently all repositories only use the basic MongoRepository methods.
Custom queries would enable more useful search and filter operations across the API.

Suggested queries to implement

• LoanRepository — findByCustomerId, findByBookId, findByReturnedFalse
• BookRepository — findByAvailableTrue
• CustomerRepository — findByCustomerId
• SectionRepository — to be decided

Note

This list is not exhaustive. Additional queries should be considered based on
what search/filter operations make sense for the API before implementation.
Expose relevant queries as new GET endpoints in the controllers.