Backend/c find implementation - #38

This pull request adds full DIMSE C-FIND (Study Root) SCP support to the DIMSE server, enabling DICOM query/retrieve operations alongside C-STORE. It introduces a new CFindService with flexible query capabilities, configuration properties, and integrates the service into the DIMSE server startup and transfer capability negotiation. Documentation and usage examples have also been updated to reflect these new features.

Major features and integration:

• Added a new CFindService class implementing Study Root C-FIND SCP, supporting standard DICOM keys, mixed free-text and keyword queries, sequence/nested keys, capped results, and proper error handling.
• Introduced DimseCFindProperties for configuring supported query levels and maximum results, with Spring Boot integration. [1] [2]
• Registered and wired up CFindService as a Spring bean, and injected it into the DimseCStoreServer and new DimseCFindSCP handler. [1] [2] [3] [4]
• Updated the DIMSE server to advertise C-FIND SCP capability with support for DATETIME and FUZZY query options in DICOM association negotiation.

Documentation and dependency updates:

• Updated backend/README.md to document C-FIND support, query behavior, supported filters, and provided example findscu commands. [1] [2] [3]
• Added dicoogle-next-sdk as a dependency for DIMSE plugin interfaces.