Upload one activity file (FIT, GPX or TCX)

One gzip NDJSON part (at most 64 MiB, never truncated) holding one activity file on one line: {"type":"fit","data":"<base64 of the file>"}, type being fit, gpx or tcx and equal to X-OHS-Type. The file is at most 20 MiB once decoded (the line format is in docs/connectors/activity-files.md; ohs import files writes it). The part is refused with 400, and nothing is stored, unless the file parses as an activity that starts at most 48 hours from now: a FIT course, settings or monitoring file, a GPX route without times or a file with a broken CRC is refused. Same headers, limits and answers as the Apple Health upload. Re-sending the same part is safe: the answer is 200 with duplicate: true. Other bytes under a (day, batch, part) already stored are a 409, and so is a part sent while this provider's data is being deleted for the user (nothing is kept; send it again once the deletion job is done). Each instance processes a bounded number of uploads at once; above it the answer is 503 with Retry-After.