feat: backfill scripts to implement core functions for already uploaded files

backfill/media_duration.py:

• ✅ Imports: from app.utils.media_duration import get_media_duration_with_remux
• ✅ Usage: Calls get_media_duration_with_remux(file_path, media_type.value)
• ✅ Storage: Uses from app.utils.hetzner_storage import get_storage_client

backfill/file_hashing.py:

• ✅ Imports: from app.utils.file_hashing import get_file_hash
• ✅ Usage: Calls get_file_hash(file_path, self.algorithm)
• ✅ Storage: Uses from app.utils.hetzner_storage import get_storage_client

backfill/snr_frequency.py:

• ✅ Imports: from app.utils.snr_frequency import calculate_snr_frequency
• ✅ Usage: Calls calculate_snr_frequency(file_path, media_type.value)
• ✅ Storage: Uses from app.utils.hetzner_storage import get_storage_client