[C#] Implement Transaction Isolation for bulk lamp move

[davideme]

Task

Implement POST /rooms/{roomId}/lamps/bulk-move with proper transaction isolation.

Endpoint

POST /rooms/{roomId}/lamps/bulk-move

{
  "lampIds": ["uuid1", "uuid2", "uuid3"],
  "targetRoomId": "target-uuid"
}

Functional Requirement

• Must be atomic - either all lamps move or none do
• Must handle concurrent requests safely - no race conditions

Implementation Hints

• Use using var transaction = await context.Database.BeginTransactionAsync()
• Consider IsolationLevel.Serializable for strict isolation
• Use ExecuteUpdateAsync for bulk updates
• Call transaction.CommitAsync() only on success

Acceptance Criteria

• All lamps move to target room on success
• No lamps move if any validation fails (atomic)
• Returns 409 if any lamp doesn't belong to source room
• Concurrent requests don't cause data corruption

Parent Issue

Relates to #289