Refactor: Move `get_recording_datetime()` and `calculate_detected_at()` to `utils/` module

[sorochak]

The get_recording_datetime() and calculate_detected_at() helpers in audio_analyzer.py are responsible for parsing and computing timestamps from filenames. These are general-purpose utilities and should be moved to a shared utils/ module to improve separation of concerns, reuse, and testability.

Tasks

• Create backend/utils/file_parsing.py
• Move get_recording_datetime() to file_parsing.py
• Move calculate_detected_at() to file_parsing.py
• Update imports in audio_analyzer.py (and any other affected files)
• Confirm all functionality and tests still work