refactor(admin): retire settings migration and compat menu demotion

Stop auto-running AKSRT_Settings_Migration, drop multi-option settings
merge and Zibll favicon aliases, remove the migration tools entry from
menus/schema, flatten admin sections so pages are not demoted into a
compat bag, and retire the migration-status payload for leftover SPA
routes after a full admin rebuild.

assets/admin/src/schema/sections/tools/index.js

@@ -5,14 +5,12 @@
 
 import { importExportSchema } from './import-export.js'
 import { healthSchema } from './health.js'
-import { migrationSchema } from './migration.js'
 import { emailRollbackSchema } from './email-rollback.js'
 import { backupSchema } from './backup.js'
 
 export const toolsSchemas = {
   'import-export': importExportSchema,
   health: healthSchema,
-  migration: migrationSchema,
   'email-rollback': emailRollbackSchema,
   backup: backupSchema,
 }