fix: match file extensions case-insensitively

[anrinion]

Summary

• FileExtension.contains() compared entry.suffix against the enum's lowercase extension values case-sensitively, so files with uppercase extensions (e.g. iPhone's default IMG_1234.MOV) were silently skipped by _list_input_directory_files, surfacing as a confusing EmptyInputDirectoryError even though a valid video was present in the input directory.
• Lowercases the input before the membership check so extension matching is case-insensitive.

Closes #199

Test plan

• Reproduced: placed an uppercase-extension video (IMG_xxxx.MOV) in input_directory, ran best_frames_extractor via docker-compose up + POST /v2/extractors/best_frames_extractor — failed with EmptyInputDirectoryError before the fix.
• Applied the fix, rebuilt, re-ran the same request — file was found, frame extracted successfully, input file renamed with frames_extracted_ prefix as expected.