Round up for stream record lookback

[uvjustin]

Proposed change

#73604 modified the stream lookback behavior to avoid duplicate segments which occur under certain conditions, allowing recordings to work ahead of #73478, but those two PRs taken together may cause stream recordings to start slightly late when those duplicate segments do not occur. For example, when lookback is small (0 <= lookback <= target_duration) and the worker is already running, the in progress segment may be in the HLSStreamOutput and not in the RecorderOutput. However, since the current code rounds down when calculating num_segments, num_segments comes out to 0 and thus none of the in progress segment is ever captured.
This PR modifies the stream recorder to always round up 1 segment when the stream is already active. While the lookback calculation is still inexact, it is better to include more data than to miss some.

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[probot-home-assistant]

Hey there @hunterjm, @allenporter, mind taking a look at this pull request as it has been labeled with an integration (stream) you are listed as a code owner for? Thanks!
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