fix(audio): add diagnostic logging for per-source EQ save flow (#2908)

* fix(audio): add diagnostic logging for per-source EQ save flow (#2849)

Add debug-level logging at three points in the per-source equalizer
save chain to help diagnose reports where per-source EQ filters are
not persisting through the Sound Card settings UI.

Backend (settings_audio.go):
- Log each source's per-source EQ state (nil/enabled/filter count)
  during every settings save, showing what the backend received.

Frontend (SoundCardCard.svelte):
- Log EQ state in handleEqualizerUpdate (toggle/filter changes)
- Log final transformed EQ in saveEdit before submitting to store

Frontend (AudioSettingsPage.svelte):
- Log each source's EQ state in updateAudioSources as data enters
  the settings store

All logging is debug-level and only visible when debug mode is
enabled, so there is no impact on production.

* fix: address review feedback

- Remove noisy handleEqualizerUpdate log (fires on every slider input)
- Gate per-source EQ logging behind EQ change detection
- Redact source names from backend logs (PII risk)

Author: tphakala

frontend/src/lib/desktop/components/forms/SoundCardCard.svelte

@@ -21,6 +21,7 @@
   import { slide } from 'svelte/transition';
   import { t } from '$lib/i18n';
   import { cn } from '$lib/utils/cn';
+  import { loggers } from '$lib/utils/logger';
   import SelectDropdown from './SelectDropdown.svelte';
   import InlineSlider from './InlineSlider.svelte';
   import QuietHoursEditor from './QuietHoursEditor.svelte';
@@ -47,6 +48,8 @@
     }>;
   }
 
+  const logger = loggers.audio;
+
   // Default model ID — BirdNET v2.4 is the built-in default
   const DEFAULT_MODEL_ID = 'birdnet';
 
@@ -156,6 +159,16 @@
       quietHours: editQuietHours,
     };
 
+    logger.debug('SoundCardCard saveEdit', {
+      component: 'SoundCardCard',
+      action: 'saveEdit',
+      sourceName: trimmedName,
+      eqEnabled: editEqualizer.enabled,
+      eqFilterCount: editEqualizer.filters.length,
+      transformedEqPresent: transformedEqualizer !== undefined,
+      transformedFilterCount: transformedEqualizer?.filters?.length ?? 0,
+    });
+
     const success = onUpdate(updated);
     if (success) {
       isEditing = false;

frontend/src/lib/desktop/features/settings/pages/AudioSettingsPage.svelte

@@ -415,6 +415,15 @@
 
   // Update handlers
   function updateAudioSources(sources: AudioSourceConfig[]) {
+    for (const src of sources) {
+      logger.debug('updateAudioSources', {
+        component: 'AudioSettingsPage',
+        sourceName: src.name,
+        eqPresent: src.equalizer !== undefined,
+        eqEnabled: src.equalizer?.enabled,
+        eqFilterCount: src.equalizer?.filters?.length ?? 0,
+      });
+    }
     settingsActions.updateSection('realtime', {
       audio: { ...$audioSettings!, sources },
     });

internal/api/v2/settings_audio.go

@@ -269,8 +269,19 @@ func (c *Controller) handleAudioSettingsChanges(oldSettings, currentSettings *co
 	perSourceEQChanged := perSourceEqualizerChanged(oldSettings, currentSettings)
 	perStreamEQChanged := perStreamEqualizerChanged(oldSettings, currentSettings)
 	nameChanged := srcNameChanged || strmNameChanged
+
 	if globalEQChanged || perSourceEQChanged || perStreamEQChanged || nameChanged {
-		c.Debug("Audio equalizer settings changed, updating filter chains")
+		c.Debug("Audio equalizer settings changed (global=%v, perSource=%v, perStream=%v, nameChanged=%v)",
+			globalEQChanged, perSourceEQChanged, perStreamEQChanged, nameChanged)
+		for i := range currentSettings.Realtime.Audio.Sources {
+			src := &currentSettings.Realtime.Audio.Sources[i]
+			if src.Equalizer != nil {
+				c.Debug("Source %d: per-source EQ enabled=%v, filters=%d",
+					i, src.Equalizer.Enabled, len(src.Equalizer.Filters))
+			} else {
+				c.Debug("Source %d: per-source EQ is nil (using global)", i)
+			}
+		}
 		if err := c.handleEqualizerChange(currentSettings); err != nil {
 			c.Debug("Failed to update EQ filter chains: %v", err)
 		}