fix: guard RTF division in debug log for zero-duration audio

[Archlie]

Fixes #667

Problem

Uploading a well-formed but zero-duration audio file (e.g. a header-only WAV with 0 samples) to POST /v1/audio/transcriptions returns HTTP 500 instead of the pre-0.9.0 clean 200 with empty text.

Root cause: the RTF debug log line in audio_file_dependency uses an eager f-string:

logger.debug(f"Decoded {audio.duration}s of audio in {elapsed:.5f}s (RTF: {elapsed / audio.duration})")

Because f-strings evaluate eagerly, elapsed / audio.duration runs even when debug logging is disabled (log_level does not mitigate it). audio.duration == 0 is reachable: a well-formed container that decodes to an empty sample array makes decode_audio() succeed (so the av.error.* handlers don't fire) and returns a zero-length result. The ZeroDivisionError is then caught by the generic except Exception handler, which turns it into a 500.

Fix

Guard the division with audio.duration > 0. Zero-duration uploads now flow through to the normal empty-transcription path, restoring the pre-rc behavior; normal files still log RTF as before.

Test

Added tests/dependencies_test.py with a mock-based regression test that feeds a zero-duration sample array through audio_file_dependency and asserts no crash and duration == 0.0.