Add OrderQueueService with estimated wait-time endpoints

[Copilot]

Adds an in-memory order queue to the backend so store staff and waiting customers can see current queue depth and estimated wait times based on real order history.

New

• OrderQueueEntry model — tracks OrderId, QueuedAt, CompletedAt, Status (Queued / InProgress / Completed), and a computed PrepTimeMinutes
• OrderQueueService (singleton) — thread-safe ConcurrentDictionary-backed queue; exposes:
  • EnqueueOrder / CompleteOrder
  • GetPendingOrderCount / GetAveragePrepTimeMinutes / GetEstimatedWaitMinutes
• OrderQueueController — three endpoints:

Method	Route	Description
GET	/api/orderqueue/status	Pending count, estimated wait, avg prep time
GET	/api/orderqueue/average-prep-time	Avg prep time; 204 until first completion
POST	/api/orderqueue/{id}/complete	Mark order done, record its prep time

• QueueStatusResponse / AveragePrepTimeResponse DTOs in Models/

Modified

• OrderService.CreateOrderAsync — calls EnqueueOrder after persisting each new order
• Program.cs — registers OrderQueueService as a singleton

Estimation logic

estimatedWait = pendingOrderCount × avgPrepTimeMinutes

Defaults to 5 min/order until completion history is available.

Original prompt

Add a small queue implementation to the backend to track order preparation times for store management. This will allow people that are waiting for their orders to see estimated wait times based on current order volume.

• Show me the design and implementation plan Todo list.
• Implement the backend OrderQueueService to track order times.
• Create API endpoints to get current queue status and average preparation times.

The user has attached the following file paths as relevant context:

• .github\instructions\copilot-instructions.instructions.md

Created from VS Code.

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