fix: validate invalid dicom uploads before processing

[AlwinXue]

Hi mentors! 👋

This PR addresses the missing DICOM pre-flight validation reported in Issue #31.

The Problem:
The /api/report/process/dicom endpoint returned a 500 Server Error when users uploaded files that were not valid DICOMs. The route saved the uploads and immediately called get_bids_metadata(), which eventually failed because the upload directory contained no readable DICOM headers. That failure was then surfaced as a generic 500 instead of a clear client error.

The Solution:
Pre-flight Validation: Added a validation step after saving uploads that calls get_dicom_header() on the uploaded directory before report generation starts.
Clear Client Error: If no valid DICOM files are found, the route now cleans up the temporary upload directory and returns 400 Bad Request with No valid DICOM files provided.
Test Alignment: Updated the invalid-file backend test to hit the correct /api/report/process/dicom route and assert the deterministic 400 response.

Validation:
Ran python -m pytest tests/test_report.py -q -k "test_get_report_dicom_no_files or test_get_report_dicom_with_invalid_file" (with DYLD_FALLBACK_LIBRARY_PATH=/opt/homebrew/lib) and both DICOM endpoint tests passed.
Note: the full backend test file still contains unrelated failures tied to other open issues, so validation here is scoped to Issue #31.

Fixes #31