Fix break classification for zero-served and short episodes

Finding 1: Treat zero or invalid servedMs as unjudgeable, falling back to
advertising classification to match the constraint that anything unjudgeable
must be conservative (ffprobe duration probe returning 0 is plausible upstream
defect).

Finding 2: Use proportional edge windows for short episodes (< 240s) to avoid
misclassifying interior and closing breaks as intro when opening and closing
windows overlap. For a short episode with breaks at 0s, 50s, 90s in a 100s
episode, classify them correctly as intro, cue, outro instead of all intro.

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