feat(schema): consolidate the 2.x migration chain into one guarded base migration

A single consolidation migration replaces the 150 removed historical files.
It decides — from reads alone — whether to build the boundary schema fresh,
skip on a fully-migrated 2.x database, or refuse a partial or inconsistent
history untouched. Fresh installs run it first and the live v3 chain after;
pre-2.x installs are directed through the latest 2.x release. The same
verdict backs a self-updater preflight so an unsafe upgrade is refused
before any file is copied. Schema verified identical to the boundary
fixtures on SQLite, MariaDB and PostgreSQL.

app/Platform/Operations/Http/Admin/UpdateController.php

@@ -95,11 +95,25 @@ class UpdateController extends Controller
         return response()->json(Updater::cleanStaleFiles());
     }
 
+    /**
+     * Run the pending migrations, unless the schema guard refuses first.
+     *
+     * A refusal is the operator's problem to fix, not a crash: it comes back
+     * with the guard's explanation in the same shape the unzip step uses for
+     * its own failures.
+     */
     public function migrate(Request $request): JsonResponse
     {
         $this->authorizeUpdates();
 
-        Updater::migrateUpdate();
+        try {
+            Updater::migrateUpdate();
+        } catch (Exception $failure) {
+            return response()->json([
+                'success' => false,
+                'error' => $failure->getMessage(),
+            ], 500);
+        }
 
         return response()->json(['success' => true]);
     }