chore: [env] .env.example 업데이트 및 .gitignore 정리

- .env.example을 SAM 프로젝트 실제 키 구조로 업데이트
- .gitignore에 !.env.example 예외 추가
- GCS_* 중복 키 제거, Gemini/Claude/Vertex 키 섹션 추가

database/migrations/2026_02_21_100000_add_detail_fields_to_cards_table.php

@@ -1,46 +0,0 @@
-<?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::table('cards', function (Blueprint $table) {
-            $table->string('card_type', 50)->nullable()->after('card_company')->comment('카드 종류 (corporate_1, personal 등)');
-            $table->string('alias', 100)->nullable()->after('card_name')->comment('카드 별칭');
-            $table->text('cvc_encrypted')->nullable()->after('card_password_encrypted')->comment('암호화된 CVC/CVV');
-            $table->unsignedTinyInteger('payment_day')->nullable()->after('cvc_encrypted')->comment('결제일 (1-31)');
-            $table->decimal('total_limit', 15, 2)->nullable()->after('payment_day')->comment('총 한도');
-            $table->decimal('used_amount', 15, 2)->nullable()->after('total_limit')->comment('사용 금액');
-            $table->decimal('remaining_limit', 15, 2)->nullable()->after('used_amount')->comment('잔여 한도');
-            $table->boolean('is_manual')->default(false)->after('status')->comment('수기 등록 여부');
-            $table->text('memo')->nullable()->after('assigned_user_id')->comment('메모');
-        });
-    }
-
-    /**
-     * Reverse the migrations.
-     */
-    public function down(): void
-    {
-        Schema::table('cards', function (Blueprint $table) {
-            $table->dropColumn([
-                'card_type',
-                'alias',
-                'cvc_encrypted',
-                'payment_day',
-                'total_limit',
-                'used_amount',
-                'remaining_limit',
-                'is_manual',
-                'memo',
-            ]);
-        });
-    }
-};