feat: H-2 재고 현황 API 구현

- StockController: 재고 조회 및 통계 API
- StockService: 재고 비즈니스 로직
- Stock, StockLot 모델: 재고/로트 관리
- Swagger 문서화
- stocks, stock_lots 테이블 마이그레이션

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/Models/Tenants/StockLot.php

@@ -0,0 +1,111 @@
+<?php
+
+namespace App\Models\Tenants;
+
+use App\Traits\BelongsToTenant;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\SoftDeletes;
+
+class StockLot extends Model
+{
+    use BelongsToTenant, SoftDeletes;
+
+    protected $fillable = [
+        'tenant_id',
+        'stock_id',
+        'lot_no',
+        'fifo_order',
+        'receipt_date',
+        'qty',
+        'reserved_qty',
+        'available_qty',
+        'unit',
+        'supplier',
+        'supplier_lot',
+        'po_number',
+        'location',
+        'status',
+        'receiving_id',
+        'created_by',
+        'updated_by',
+        'deleted_by',
+    ];
+
+    protected $casts = [
+        'fifo_order' => 'integer',
+        'receipt_date' => 'date',
+        'qty' => 'decimal:3',
+        'reserved_qty' => 'decimal:3',
+        'available_qty' => 'decimal:3',
+        'stock_id' => 'integer',
+        'receiving_id' => 'integer',
+    ];
+
+    /**
+     * LOT 상태 목록
+     */
+    public const STATUSES = [
+        'available' => '사용가능',
+        'reserved' => '예약됨',
+        'used' => '사용완료',
+    ];
+
+    /**
+     * 재고 관계
+     */
+    public function stock(): BelongsTo
+    {
+        return $this->belongsTo(Stock::class);
+    }
+
+    /**
+     * 입고 관계
+     */
+    public function receiving(): BelongsTo
+    {
+        return $this->belongsTo(Receiving::class);
+    }
+
+    /**
+     * 생성자 관계
+     */
+    public function creator(): BelongsTo
+    {
+        return $this->belongsTo(\App\Models\Members\User::class, 'created_by');
+    }
+
+    /**
+     * 상태 라벨
+     */
+    public function getStatusLabelAttribute(): string
+    {
+        return self::STATUSES[$this->status] ?? $this->status;
+    }
+
+    /**
+     * 경과일
+     */
+    public function getDaysElapsedAttribute(): int
+    {
+        return $this->receipt_date->diffInDays(now());
+    }
+
+    /**
+     * 가용 수량 업데이트
+     */
+    public function updateAvailableQty(): void
+    {
+        $this->available_qty = $this->qty - $this->reserved_qty;
+
+        if ($this->available_qty <= 0 && $this->qty <= 0) {
+            $this->status = 'used';
+        } elseif ($this->reserved_qty > 0) {
+            $this->status = 'reserved';
+        } else {
+            $this->status = 'available';
+        }
+
+        $this->save();
+    }
+}