lab_beatgrid

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Measure the TEMPO of the sequence's bound music track and store the beat grid — opens the real file with ffmpeg and analyses it, no guessing and no provider, so it costs nothing but a few seconds. Writes `bpm` and `metadata.beat_grid` on the audio row; it does NOT touch the timeline (`timeline_geaendert: false`). Deterministic and cached: a second call returns the same numbers without downloading again (pass `neu_messen` to force). READ `confidence` AND `verlaesslich` BEFORE YOU CUT. Below the threshold (0.35) lab_apply_rhythm computes durations but deliberately does NOT snap them to the grid — that is a decision, not a failure. Soft material measures badly: a real swing track came out at 0.164, an ordered hard pulse (techno, 128 bpm) at 0.621. Half and double tempo are the classic misreads, so `bpm_alternativen` is part of the answer. `frame_konflikt` reports the collision between beat grid and frame grid, for BOTH renderers: `export` (24 fps, the delivered MP4) and `murnau` (25 fps, the editor timeline). Read `frames_pro_schlag` as a DEVIATION, not a count: it is how far one beat sits from a whole frame, between 0 and 0.5. 0 means beat and frame coincide (100, 125 and 150 bpm at 25 fps do); anything above means beat-accurate and frame-accurate cannot both hold, and the rounding error accumulates across clips — measured on a finished MP4 at 128 bpm: up to 64.7 ms off the audible beat although every cut point had snapped. Shown here, not fixed: that decision belongs to a human. Refusals that name their own remedy: 409 without a bound music track (run lab_bind_audio), 409 when the track is pending (poll lab_list_audio), 502/504 when the audio host is unreachable or slow, 422 when the file carries no usable audio, 503 when ffmpeg is missing.