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/Repositories/IAgentStateRepository.cs

@@ -0,0 +1,38 @@
+using ControlCenter.Models;
+
+namespace ControlCenter.Repositories;
+
+/// <summary>
+/// Repository interface for querying and updating agent state.
+///
+/// <para>Provides the data-access contract used by controllers,
+/// background services, and SignalR hubs to read and mutate
+/// persistent agent state.</para>
+///
+/// <para>Implementation should use <see cref="Data.ControlCenterDbContext"/>
+/// via EF Core with PostgreSQL (snake_case columns).</para>
+/// </summary>
+public interface IAgentStateRepository
+{
+    /// <summary>
+    /// Returns all agent states from the database.
+    /// </summary>
+    /// <returns>A collection of all <see cref="AgentState"/> records.</returns>
+    Task<IEnumerable<AgentState>> GetAllAsync();
+
+    /// <summary>
+    /// Finds an agent state by its session key.
+    /// </summary>
+    /// <param name="sessionKey">The full session key, e.g. "agent:dex:telegram:direct:...".</param>
+    /// <returns>The matching <see cref="AgentState"/>, or null if not found.</returns>
+    Task<AgentState?> GetBySessionKeyAsync(string sessionKey);
+
+    /// <summary>
+    /// Updates the status of an agent state record.
+    /// Also updates <c>LastActivity</c> to <see cref="DateTime.UtcNow"/>.
+    /// </summary>
+    /// <param name="id">The unique identifier of the agent state record.</param>
+    /// <param name="status">The new status value ("active", "idle", "thinking", "error").</param>
+    /// <returns>The updated <see cref="AgentState"/>, or null if the record was not found.</returns>
+    Task<AgentState?> UpdateStatusAsync(Guid id, string status);
+}