feat: Implement inventory service with stock reservation

[rezadrian01]

Summary

• Full inventory service implementation (port 8085) managing stock levels for products
• Shares products_db with product-service — no separate database, no AutoMigrate (product-service owns the inventory table)
• Consumes order.created → reserves stock via ReservedQuantity += n (optimistic atomic UPDATE)
• Consumes order.cancelled → releases reservation via GREATEST(reserved_quantity - n, 0)
• Publishes inventory.updated on every stock change; publishes inventory.low_stock when available stock ≤ 10
• Admin-only HTTP endpoints; auth enforced at the gateway (requireRole("admin"))

Changes

New service — services/inventory-service/

• Domain: Inventory entity (mirrors product-service table schema exactly), AvailableQuantity() helper, LowStockThreshold = 10, UpdateInventoryRequest, OrderCreatedEvent with json:"id" (matches order-service's Order.ID tag)
• Repository: GetByProductID, SetTotalQuantity (PostgreSQL upsert via INSERT ... ON CONFLICT DO UPDATE), ReserveStock (atomic WHERE clause prevents overbooking, distinguishes ErrInventoryNotFound vs ErrInsufficientStock), ReleaseStock (GREATEST clamp prevents negative reservation)
• Cache: inventory:<product_id> key, 5 min TTL (shorter because stock changes on every order)
• Kafka publisher: writes to inventory.updated and inventory.low_stock
• Kafka consumer: multi-topic struct with two kafka.Reader instances — one per topic (order.created, order.cancelled); Start() launches one goroutine per reader; handleMessage switches on topic
• Service: HandleOrderCreated loops items, reserves each, logs and continues on failure (partial reservation acceptable); HandleOrderCancelled releases each item's reservation
• Handler + route: GET /inventory/:product_id, PUT /inventory/:product_id; no auth helper needed (gateway handles it)
• cmd: no runMigrations — product-service owns the table; Kafka consumer uses group inventory-service-orders

docker-compose.yml

• Added REDIS_URL, KAFKA_BROKERS to inventory-service environment
• Added kafka: condition: service_healthy to inventory-service.depends_on

Test plan

• docker-compose up inventory-service products-db redis kafka starts without errors
• GET /health returns {"status":"healthy","service":"inventory-service"}
• PUT /api/inventory/:product_id (as admin) sets total stock and returns updated inventory
• GET /api/inventory/:product_id (as admin) returns stock levels with computed available_quantity
• Placing an order triggers order.created → reserved_quantity increments for each item
• Cancelling an order triggers order.cancelled → reserved_quantity decrements for each item
• Setting stock below threshold publishes inventory.low_stock event (visible in Kafka UI)

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