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/Data/Configurations/AgentStateConfiguration.cs

@@ -0,0 +1,69 @@
+using ControlCenter.Models;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace ControlCenter.Data.Configurations;
+
+/// <summary>
+/// EF Core entity configuration for <see cref="AgentState"/>.
+///
+/// <para>Maps to the <c>agents</c> table with snake_case column naming,
+/// consistent with the Extrudex project conventions.</para>
+///
+/// <para>CUB-56 will create the migration; this configuration only
+/// defines the entity-to-table mapping and constraints.</para>
+/// </summary>
+public class AgentStateConfiguration : IEntityTypeConfiguration<AgentState>
+{
+    public void Configure(EntityTypeBuilder<AgentState> builder)
+    {
+        // Table name
+        builder.ToTable("agents");
+
+        // Primary key
+        builder.HasKey(a => a.Id);
+
+        // Properties with snake_case column names
+        builder.Property(a => a.Id)
+            .HasColumnName("id")
+            .ValueGeneratedOnAdd();
+
+        builder.Property(a => a.Status)
+            .HasColumnName("status")
+            .IsRequired()
+            .HasMaxLength(20)
+            .HasDefaultValue("idle");
+
+        builder.Property(a => a.Task)
+            .HasColumnName("task")
+            .IsRequired(false)
+            .HasMaxLength(500);
+
+        builder.Property(a => a.Progress)
+            .HasColumnName("progress")
+            .IsRequired(false);
+
+        builder.Property(a => a.SessionKey)
+            .HasColumnName("session_key")
+            .IsRequired()
+            .HasMaxLength(255);
+
+        builder.Property(a => a.Channel)
+            .HasColumnName("channel")
+            .IsRequired()
+            .HasMaxLength(50);
+
+        builder.Property(a => a.LastActivity)
+            .HasColumnName("last_activity")
+            .IsRequired()
+            .HasDefaultValueSql("NOW()");
+
+        // Indexes
+        builder.HasIndex(a => a.SessionKey)
+            .IsUnique()
+            .HasDatabaseName("ix_agents_session_key");
+
+        builder.HasIndex(a => a.Status)
+            .HasDatabaseName("ix_agents_status");
+    }
+}