Add DB trigger mode for session grouping (SESSION_TRIGGER)

Lars Dalbyrequested to merge
dev into main

When SESSION_TRIGGER=true (or session_trigger=true via API), the pipeline skips client-side session grouping and lets the DB trigger (migration 2034) auto-create sessions and assign sessionid on INSERT. This simplifies the pipeline from 12 steps to 5 when writing to the database.

New functions: et_build_sourceimages(type=NULL) for per-image type detection, et_filter_stray_sourceimages() for sourceimages-schema stray filtering, et_db_get_sessions() for querying trigger-created sessions.

Adapt to trimmed down IoT table.

Trim CLAUDE.md

Add mothbox filename parsing.

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