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/routers/settings.py

@@ -267,6 +267,7 @@ class PreferencesUpdate(BaseModel):
     calibration_warning_days: Optional[int] = None
     status_ok_threshold_hours: Optional[int] = None
     status_pending_threshold_hours: Optional[int] = None
+    mic_unit_pref: Optional[str] = None
 
 
 @router.get("/preferences")
@@ -293,6 +294,7 @@ def get_preferences(db: Session = Depends(get_db)):
         "calibration_warning_days": prefs.calibration_warning_days,
         "status_ok_threshold_hours": prefs.status_ok_threshold_hours,
         "status_pending_threshold_hours": prefs.status_pending_threshold_hours,
+        "mic_unit_pref": prefs.mic_unit_pref or "dBL",
         "updated_at": prefs.updated_at.isoformat() if prefs.updated_at else None
     }
 
@@ -334,6 +336,7 @@ def update_preferences(
         "calibration_warning_days": prefs.calibration_warning_days,
         "status_ok_threshold_hours": prefs.status_ok_threshold_hours,
         "status_pending_threshold_hours": prefs.status_pending_threshold_hours,
+        "mic_unit_pref": prefs.mic_unit_pref or "dBL",
         "updated_at": prefs.updated_at.isoformat() if prefs.updated_at else None
     }