settings: add mic_unit_pref for event-report chart

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>

backend/models.py

@@ -135,6 +135,9 @@ class UserPreferences(Base):
     calibration_warning_days = Column(Integer, default=30)
     status_ok_threshold_hours = Column(Integer, default=12)
     status_pending_threshold_hours = Column(Integer, default=24)
+    # Mic display units on the event-report waveform chart only — peaks
+    # and KPI tiles elsewhere are always dBL.  "dBL" (default) or "psi".
+    mic_unit_pref = Column(String, default="dBL")
     updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)