fix: enforce connection and read timeouts on external audio downloads

[iyedmdimegh]

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[iyedmdimegh]

While going through the codebase as part of my GSoC 2026 application,
I noticed that external audio downloads in AudioDataLayer had no timeout
set on the requests.get() call so, as a result, on a slow or unresponsive server this
can block the Flask worker thread indefinitely, or freeze the API.

The fix uses a (10, 300) timeout tuple: 10s on the connection so
unreachable servers fail fast, and 300s on the read to give large
audio/video files enough time to actually finish downloading without
triggering a false timeout.

I also added a dedicated Timeout exception handler that raises a clear RuntimeError including the URL.

On the testing side, I added test_fetch_audio_from_url_timeout to verify both that the timeout tuple is passed to requests.get and that the expected RuntimeError message is raised on timeout.

Ran the full test suite: 93 passed, 4 warnings.