Name the COCO frame rate annotation_fps

[mattdawkins]

Follow-up to #1861. The rate written on the COCO video entry is the rate detections were produced at — the downsampled -frate, not the video's native rate — so VIAME now writes it as annotation_fps (VIAME/VIAME@9b1227cc6). This renames the key on DIVE's side to match.

Without this, #1861's importer reads videos[].fps while current VIAME writes videos[].annotation_fps, so the frame rate does not survive a VIAME to DIVE import.

• server: frame_rate_from_coco() reads it, export_dive_as_coco() writes it
• desktop: frameRateFromDocument() reads it, serializeFile writes it
• docs/DataFormats.md, including the section anchor and the two links to it
• DIVE's own dataset metadata key is untouched and stays fps; only the COCO wire spelling changes

Verified against real process_video.py output: desktop parseFile gives meta.fps = 5 for a video and undefined for an image list; server import gives 5.0, export then writes [{'id': 1, 'name': 'clip', 'annotation_fps': 5.0}], and re-import gives 5.0 back.

Not run: vitest, eslint and tsc — no node_modules in this checkout. Server-side checks were executed directly.