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/AgentStateRepository.cs

@@ -0,0 +1,62 @@
+using ControlCenter.Data;
+using ControlCenter.Models;
+using Microsoft.EntityFrameworkCore;
+
+namespace ControlCenter.Repositories;
+
+/// <summary>
+/// EF Core implementation of <see cref="IAgentStateRepository"/>.
+///
+/// <para>Queries the <c>agents</c> table via <see cref="ControlCenterDbContext"/>
+/// using snake_case column mappings defined in the <see cref="Data.Configurations.AgentStateConfiguration"/>.
+/// All write operations call <c>SaveChangesAsync</c> immediately.</para>
+/// </summary>
+public class AgentStateRepository : IAgentStateRepository
+{
+    private readonly ControlCenterDbContext _context;
+    private readonly ILogger<AgentStateRepository> _logger;
+
+    public AgentStateRepository(
+        ControlCenterDbContext context,
+        ILogger<AgentStateRepository> logger)
+    {
+        _context = context;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public async Task<IEnumerable<AgentState>> GetAllAsync()
+    {
+        _logger.LogDebug("Fetching all agent states");
+        return await _context.AgentStates.ToListAsync();
+    }
+
+    /// <inheritdoc />
+    public async Task<AgentState?> GetBySessionKeyAsync(string sessionKey)
+    {
+        _logger.LogDebug("Looking up agent state by session key: {SessionKey}", sessionKey);
+        return await _context.AgentStates
+            .FirstOrDefaultAsync(a => a.SessionKey == sessionKey);
+    }
+
+    /// <inheritdoc />
+    public async Task<AgentState?> UpdateStatusAsync(Guid id, string status)
+    {
+        _logger.LogDebug("Updating agent status: {Id} → {Status}", id, status);
+
+        var entity = await _context.AgentStates.FindAsync(id);
+        if (entity is null)
+        {
+            _logger.LogWarning("Agent state not found: {Id}", id);
+            return null;
+        }
+
+        entity.Status = status;
+        entity.LastActivity = DateTime.UtcNow;
+
+        _context.AgentStates.Update(entity);
+        await _context.SaveChangesAsync();
+
+        return entity;
+    }
+}