fix: JSON serialize favicon and logo URLs for PostgreSQL storage

Fixes #84

The favicon and logo upload endpoints were storing URL paths directly
without JSON.stringify(), causing PostgreSQL JSON validation errors
("Token '/' is invalid") since paths like "/uploads/favicons/..."
are not valid JSON.

Applied JSON.stringify() to:
- branding_logo_url setting (lines 358, 364)
- branding_favicon_url setting (lines 894, 900)

backend/src/routes/adminSettings.js

@@ -355,13 +355,13 @@ router.post('/logo', adminAuth, requirePermission('settings.edit'), upload.singl
     await db('app_settings')
       .insert({
         setting_key: 'branding_logo_url',
-        setting_value: publicPath,
+        setting_value: JSON.stringify(publicPath),
         setting_type: 'branding',
         updated_at: new Date()
       })
       .onConflict('setting_key')
       .merge({
-        setting_value: publicPath,
+        setting_value: JSON.stringify(publicPath),
         updated_at: new Date()
       });
 
@@ -891,13 +891,13 @@ router.post('/favicon', adminAuth, requirePermission('settings.edit'), faviconUp
     await db('app_settings')
       .insert({
         setting_key: 'branding_favicon_url',
-        setting_value: faviconUrl,
+        setting_value: JSON.stringify(faviconUrl),
         setting_type: 'branding',
         updated_at: new Date()
       })
       .onConflict('setting_key')
       .merge({
-        setting_value: faviconUrl,
+        setting_value: JSON.stringify(faviconUrl),
         updated_at: new Date()
       });