feat(CUB-54): implement Agent State Repository with EF Core

- Add AgentState read model (Models/AgentState.cs)
- Add IAgentStateRepository interface with GetAllAsync,
  GetBySessionKeyAsync, UpdateStatusAsync
- Add AgentStateRepository EF Core implementation
  mapping Agent entity → AgentState model
- Register IAgentStateRepository in DI (Program.cs)
- Exclude ControlCenter sub-project from Api compilation

Build: 0 warnings, 0 errors

backend/Program.cs

@@ -1,5 +1,6 @@
 using ControlCenter.Api.Data;
 using ControlCenter.Api.Hubs;
+using ControlCenter.Api.Repositories;
 using Microsoft.EntityFrameworkCore;
 
 var builder = WebApplication.CreateBuilder(args);
@@ -10,6 +11,9 @@ builder.Services.AddOpenApi();
 // Register SignalR for real-time agent status updates
 builder.Services.AddSignalR();
 
+// Register Agent State Repository
+builder.Services.AddScoped<IAgentStateRepository, AgentStateRepository>();
+
 // Register DbContext with PostgreSQL
 builder.Services.AddDbContext<AppDbContext>(options =>
 {