[C#] Implement Repeated Reads Solution with request-scoped caching

[davideme]

Task

Implement POST /lamps/bulk-toggle with request-scoped caching to avoid querying the same room multiple times.

Endpoint

POST /lamps/bulk-toggle

{
  "lampIds": ["uuid1", "uuid2", "uuid3"],
  "status": true
}

Functional Requirement

Must not query the same room multiple times even if multiple lamps belong to the same room.

Implementation Hints

• Use a Dictionary<Guid, Room> as request-scoped cache
• EF Core's change tracker may already cache entities within a DbContext scope
• Verify behavior and add explicit caching if needed

Acceptance Criteria

• Bulk toggle updates all specified lamps
• Room validation occurs for each lamp (room must be active)
• Same room is only queried once per request (verify with EF logging)
• Returns 422 if any lamp belongs to deleted room

Parent Issue

Relates to #289