feat: Sistema de Usuarios - Backend CRUD + JWT Auth (Issue #1)

Implementación fundacional del proyecto PruebaGentle:
- Arquitectura Clean/Hexagonal: Core, Infrastructure, API
- 6 Stored Procedures para CRUD + autenticación
- JWT authentication con BCrypt password hashing
- Docker Compose (SQL Server + Backend)
- Solución .NET 10 con Dapper + SqlClient

Closes #1

Backend/PruebaGentle.API/Program.cs

@@ -0,0 +1,74 @@
+using System.Text;
+using Microsoft.AspNetCore.Authentication.JwtBearer;
+using Microsoft.IdentityModel.Tokens;
+using PruebaGentle.Core.Config;
+using PruebaGentle.Core.Interfaces;
+using PruebaGentle.Infrastructure.Repositories;
+using PruebaGentle.Infrastructure.Services;
+
+var builder = WebApplication.CreateBuilder(args);
+
+// Bind JwtSettings
+builder.Services.Configure<JwtSettings>(
+    builder.Configuration.GetSection("JwtSettings"));
+
+// Dependency Injection
+builder.Services.AddScoped<IUserRepository, UserRepository>();
+builder.Services.AddSingleton<IPasswordHasher, PasswordHasher>();
+
+// JWT Authentication
+var jwtSettings = builder.Configuration.GetSection("JwtSettings");
+var secretKey = jwtSettings["Secret"] ?? throw new InvalidOperationException("JWT Secret not configured.");
+
+builder.Services.AddAuthentication(options =>
+{
+    options.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
+    options.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
+})
+.AddJwtBearer(options =>
+{
+    options.TokenValidationParameters = new TokenValidationParameters
+    {
+        ValidateIssuer = true,
+        ValidateAudience = true,
+        ValidateLifetime = true,
+        ValidateIssuerSigningKey = true,
+        ValidIssuer = "PruebaGentle",
+        ValidAudience = "PruebaGentle",
+        IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(secretKey))
+    };
+});
+
+builder.Services.AddAuthorization();
+
+// Controllers
+builder.Services.AddControllers();
+
+// OpenAPI (native .NET 10)
+builder.Services.AddOpenApi();
+
+// CORS
+builder.Services.AddCors(options =>
+{
+    options.AddDefaultPolicy(policy =>
+    {
+        policy.AllowAnyOrigin()
+              .AllowAnyMethod()
+              .AllowAnyHeader();
+    });
+});
+
+var app = builder.Build();
+
+// Middleware pipeline
+if (app.Environment.IsDevelopment())
+{
+    app.MapOpenApi();
+}
+
+app.UseCors();
+app.UseAuthentication();
+app.UseAuthorization();
+app.MapControllers();
+
+app.Run();