CUB-54: implement AgentState entity, repository, and DI registration

- Add AgentState entity mapping to 'agents' table (snake_case columns)
- Add IAgentStateRepository interface (GetAllAsync, GetBySessionKeyAsync, UpdateStatusAsync)
- Add AgentStateRepository with EF Core implementation
- Add ControlCenterDbContext with ApplyConfigurationsFromAssembly
- Add AgentStateConfiguration with snake_case column mappings and indexes
- Register DbContext (Npgsql) and repository in Program.cs DI
- Add ConnectionStrings to appsettings.json
- Add EF Core 9.0.7 and Npgsql.EntityFrameworkCore.PostgreSQL 9.0.4 packages

backend/ControlCenter/Program.cs

@@ -1,6 +1,9 @@
 using System.Reflection;
+using ControlCenter.Data;
 using ControlCenter.Hubs;
+using ControlCenter.Repositories;
 using ControlCenter.Services;
+using Microsoft.EntityFrameworkCore;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -25,6 +28,19 @@ builder.Services.AddSwaggerGen(c =>
     }
 });
 
+// ── Database ────────────────────────────────────────────────
+// PostgreSQL via EF Core with Npgsql.
+// Connection string from appsettings.json or environment variable.
+builder.Services.AddDbContext<ControlCenterDbContext>(options =>
+    options.UseNpgsql(
+        builder.Configuration.GetConnectionString("ControlCenterDb")
+            ?? throw new InvalidOperationException(
+                "Connection string 'ControlCenterDb' not found. " +
+                "Add it to appsettings.json or set the environment variable.")));
+
+// ── Repositories ────────────────────────────────────────────
+builder.Services.AddScoped<IAgentStateRepository, AgentStateRepository>();
+
 // ── CORS (kiosk + remote browser) ─────────────────────────
 // The Control Center frontend runs on a different origin than the backend.
 // SignalR requires credentials for WebSocket transport, so we use