S2a: purge legacy Form Builder PHP code and routes

api/database/migrations/2026_04_20_100000_drop_remaining_legacy_registration_tables.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Support\Facades\Schema;
+
+/**
+ * S2a — drops the three remaining legacy registration tables now that every
+ * PHP caller (controllers / services / requests / resources / policies /
+ * models / tests) has been purged. The forms:migrate-legacy-data command
+ * self-skips when these tables are absent, so environments with real
+ * legacy data can still run the migration command BEFORE running this
+ * drop migration to move their data onto the new form_* tables.
+ *
+ * down() is a hard failure: restoring these tables and repopulating them
+ * from the new form_* structure would be a lossy, non-trivial conversion
+ * and there is no supported path back. Use migrate:fresh + restore from
+ * backup if you genuinely need the legacy state.
+ */
+return new class extends Migration
+{
+    public function up(): void
+    {
+        // person_field_values has a FK to registration_form_fields; drop
+        // it first to avoid constraint ordering issues.
+        Schema::dropIfExists('person_field_values');
+        Schema::dropIfExists('registration_form_fields');
+        Schema::dropIfExists('registration_field_templates');
+    }
+
+    public function down(): void
+    {
+        throw new \RuntimeException(
+            'Legacy registration tables cannot be restored. Use migrate:fresh '
+            .'or restore from a pre-S2a backup.'
+        );
+    }
+};