fix: align fileNames hidden input with backend DTO and support multiple file uploads

[coderabbitai]

Problem

The frontend in src/main/resources/static/js/script.js appends multiple <input type="hidden" name="fileNames"> elements (one per uploaded file) into #hidden-file-inputs. However, the backend DTO CreateCaseRequest exposes a single fileName (String) field, meaning the submitted fileNames values will not bind to the DTO. As a result, uploaded files are persisted to S3 but never associated with the created case — causing orphaned S3 objects.

Raised in PR #130 review comment: #130 (comment)

Requested by: @viktorlindell12

Options

Option A — Single file only

• Change input.name = 'fileNames' → input.name = 'fileName' in script.js
• Ensure only one hidden input is ever appended (e.g. only the last uploaded file)

Option B — Multi-file support (recommended)

• Change CreateCaseRequest.fileName (String) → fileNames (List<String>)
• Update CaseService.createTicket and S3Service to accept and persist multiple file names per case
• Keep the frontend appending multiple <input name="fileNames"> entries (Spring MVC will bind them as a list)

Impact

Until this is fixed, no uploaded file will be associated with any created ticket — all uploaded files will be orphaned in S3.

[viktorlindell12]

#132