fix: prevent infinite loop in chunk_video when overlap >= chunk_duration

[sjhddh]

Bug

In sentrysearch/chunker.py, the chunk_video function computes:

step = chunk_duration - overlap

When overlap >= chunk_duration, step is <= 0. The loop condition is while start < duration, but start += step never increases start, so the loop runs forever — spawning a new ffmpeg process on every iteration and filling /tmp until disk space is exhausted or the process is killed.

Reproduction

sentrysearch index ./footage --chunk-duration 5 --overlap 5
# hangs indefinitely, spawns ffmpeg processes, fills /tmp

Any value where --overlap >= --chunk-duration triggers this (e.g. --chunk-duration 5 --overlap 6).

Fix

Add an early guard in chunk_video() that raises ValueError before any file I/O or ffmpeg invocations:

if overlap >= chunk_duration:
    raise ValueError(
        f"overlap ({overlap}s) must be less than chunk_duration ({chunk_duration}s). "
        "When overlap >= chunk_duration the step between chunks is <= 0, "
        "causing an infinite loop."
    )

Also add a click.BadParameter check at the top of the index CLI command so the user gets a clear, immediate error message before any video processing begins.

Changes

• sentrysearch/chunker.py: validate overlap < chunk_duration at the start of chunk_video()
• sentrysearch/cli.py: validate the same constraint in the index command and surface it as a CLI error