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>

templates/settings.html

@@ -122,6 +122,21 @@
                         How often the dashboard should refresh automatically
                     </p>
                 </div>
+
+                <!-- Event-Report Mic Units -->
+                <div>
+                    <label for="mic-unit-pref" class="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
+                        Event Report — Mic Channel Units
+                    </label>
+                    <select id="mic-unit-pref"
+                            class="w-full max-w-md px-4 py-2 text-gray-900 dark:text-gray-100 bg-gray-50 dark:bg-gray-700 border border-gray-300 dark:border-gray-600 rounded-lg focus:outline-none focus:ring-2 focus:ring-seismo-orange">
+                        <option value="dBL" selected>dB(L) — sound pressure level</option>
+                        <option value="psi">psi — raw pressure</option>
+                    </select>
+                    <p class="text-xs text-gray-500 dark:text-gray-400 mt-1">
+                        Applies only to the waveform chart inside the event detail modal.  Peak values everywhere else (tables, KPIs, modal summary) stay in dB(L) regardless.
+                    </p>
+                </div>
             </div>
 
             <button onclick="saveGeneralSettings()" class="mt-6 px-6 py-3 bg-seismo-orange hover:bg-orange-600 text-white rounded-lg transition-colors">
@@ -771,6 +786,9 @@ async function loadPreferences() {
         // Load auto-refresh interval
         document.getElementById('refresh-interval').value = prefs.auto_refresh_interval || 10;
 
+        // Load event-report mic units
+        document.getElementById('mic-unit-pref').value = prefs.mic_unit_pref || 'dBL';
+
         // Load status thresholds
         document.getElementById('ok-threshold').value = prefs.status_ok_threshold_hours || 12;
         document.getElementById('pending-threshold').value = prefs.status_pending_threshold_hours || 24;
@@ -788,6 +806,7 @@ async function saveGeneralSettings() {
     const timezone = document.getElementById('timezone-select').value;
     const theme = document.querySelector('input[name="theme"]:checked').value;
     const autoRefreshInterval = parseInt(document.getElementById('refresh-interval').value);
+    const micUnitPref = document.getElementById('mic-unit-pref').value;
 
     try {
         const response = await fetch('/api/settings/preferences', {
@@ -796,7 +815,8 @@ async function saveGeneralSettings() {
             body: JSON.stringify({
                 timezone,
                 theme,
-                auto_refresh_interval: autoRefreshInterval
+                auto_refresh_interval: autoRefreshInterval,
+                mic_unit_pref: micUnitPref
             })
         });