Support multiple products in a single flash-sale event

[piush365]

Background

The current system is hardcoded to a single product (IPHONE15). A real flash-sale platform runs many products concurrently — each with its own inventory, rate limit, and possibly different per-user purchase limits.

Proposed Change

Domain model

Remove the hardcoded defaultProductId assumption. Inventory keys already follow the pattern stock:product:{productId}, so Redis is already multi-product — only the initialisation and health indicator need updating.

New endpoint (or extend existing)

Allow admins to seed inventory for any product:

POST /api/v1/admin/products/{productId}/inventory
Body: { "stock": 500 }

Secured separately (admin role, not general API key).

Per-product config

flashsale:
  products:
    IPHONE15: { initial-stock: 100, rate-limit: 3 }
    PS5:      { initial-stock: 50,  rate-limit: 1 }

Acceptance Criteria

• FlashSaleProperties extended with a products map
• InventoryConfig seeds all configured products on startup
• FlashSaleHealthIndicator reports stock for every configured product
• Integration test verifies two products can be bought independently without interference
• Admin endpoint is protected and documented in Swagger UI