The NL-43 .rnh carries no measurement timestamps, so _ingest_file_entries was stamping every session with utcnow() and no duration. Derive started_at/stopped_at/duration from the Leq .rnd 'Start Time' column when the header lacks them (interval from the .rnh, else inferred from row spacing). Adds an optional unit_id so callers that know the recording unit attribute the session at creation, and returns duration_seconds.
Side effect: NL-43 dedupe now works (it keyed on a previously-empty start_time_str). Affects all ingest paths: manual upload, FTP cycle, stop, download, and manual FTP download.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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