Implement two-phase Plaud sync with catalog matching and import pipeline
Replace the PlaudBackend stub with a full sync implementation that fetches
the Plaud API catalog, matches files against existing imports by filename,
and optionally downloads + imports new recordings through the pipeline.
- Add --sync BACKEND and --save flags to sol import CLI
- Add match_existing_imports() for filename-based matching (exact, stem, sanitized)
- Add timestamp_from_start_time() for Plaud epoch conversion
- Re-check available files on each sync to catch manual imports between syncs
- Save incremental sync state to imports/plaud.json keyed by Plaud file ID
- Add scratch/migrate_plaud_imports.py for bootstrapping state from prior imports
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>