New UserPreferences field controls the mic channel's unit on the
SFM event-detail modal's waveform chart only. "dBL" default,
"psi" alternate. Peaks everywhere else (tables, KPI tiles, modal
summary) stay in dBL regardless — this is strictly a chart-axis
preference.
Surfaced as a single dropdown on Settings → General, below the
auto-refresh interval.
Setting up the storage half ahead of the chart port in the next
commit, so the chart can read the value from /api/settings/preferences
on first render instead of needing a follow-up wiring pass.
Includes idempotent backend/migrate_add_mic_unit_pref.py for fleets
already on an older schema.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
