Adding declaration as repository component & changing primary key type

[Ludy87]

Description

• Adding the @Repository annotation: This annotation explicitly informs Spring that these interfaces are repository classes used for the data access layer.
• Changing the primary key type in UserRepository: The primary key in the User entity and the corresponding database table has been changed from String to Long. Ensure that the corresponding entity classes and database configurations are also adjusted to support this change.

Checklist:

• I have read the Contribution Guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

Contributor License Agreement

By submitting this pull request, I acknowledge and agree that my contributions will be included in Stirling-PDF and that they can be relicensed in the future under the MPL 2.0 (Mozilla Public License Version 2.0) license.

(This does not change the general open-source nature of Stirling-PDF, simply moving from one license to another license)