fix: Auto-assign when new help request created

[berat-sayin]

Related to #225

Changes:

• backend/src/modules/availability/repository.js: Added findMatchingVolunteerForRequest to find an
  available volunteer who doesn't have an active assignment and matches the request's need type.
• backend/src/modules/availability/service.js: Added and exported tryToAssignRequest to encapsulate the
  assignment logic (creating an assignment record and updating the request status to ASSIGNED).
• backend/src/modules/help-requests/service.js: Updated createMyHelpRequest to call tryToAssignRequest
  immediately after a request is successfully created. It now returns the updated request (which will
  have MATCHED status if a volunteer was found).
• Tests:
  • Added a new integration test case in
    backend/tests/integration/modules/help-requests/help-requests.integration.test.js to verify that a
    new request is auto-assigned to an already available volunteer.
  • Updated unit tests in both availability and help-requests modules to reflect the new functionality
    and ensure correct mocking.

[mehmetcangurbuz08]

Looks solid. Merging.