Comment Mentions: Added core back-end logic

- Added new user notification preference, opt-in by default
- Added parser to extract mentions from comment HTML, with tests to
  cover.
- Added notification and notification handling

Not yet tested, needs testing coverage.

database/migrations/2025_12_15_140219_create_mention_history_table.php

@@ -0,0 +1,31 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::create('mention_history', function (Blueprint $table) {
+            $table->increments('id');
+            $table->string('mentionable_type', 50)->index();
+            $table->unsignedBigInteger('mentionable_id')->index();
+            $table->unsignedInteger('from_user_id')->index();
+            $table->unsignedInteger('to_user_id');
+            $table->timestamps();
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::dropIfExists('mention_history');
+    }
+};