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/Controllers/AuthController.cs

@@ -0,0 +1,73 @@
+using System.IdentityModel.Tokens.Jwt;
+using System.Security.Claims;
+using System.Text;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.Extensions.Options;
+using Microsoft.IdentityModel.Tokens;
+using PruebaGentle.Core.Config;
+using PruebaGentle.Core.DTOs;
+using PruebaGentle.Core.Interfaces;
+
+namespace PruebaGentle.API.Controllers;
+
+[ApiController]
+[Route("api/[controller]")]
+public class AuthController : ControllerBase
+{
+    private readonly IUserRepository _userRepository;
+    private readonly IPasswordHasher _passwordHasher;
+    private readonly JwtSettings _jwtSettings;
+
+    public AuthController(
+        IUserRepository userRepository,
+        IPasswordHasher passwordHasher,
+        IOptions<JwtSettings> jwtSettings)
+    {
+        _userRepository = userRepository;
+        _passwordHasher = passwordHasher;
+        _jwtSettings = jwtSettings.Value;
+    }
+
+    [HttpPost("login")]
+    public async Task<IActionResult> Login([FromBody] LoginDto dto)
+    {
+        var user = await _userRepository.GetByUsernameAsync(dto.Username);
+        if (user == null)
+            return Unauthorized(new { error = "Credenciales inválidas." });
+
+        if (!_passwordHasher.Verify(dto.Password, user.PasswordHash))
+            return Unauthorized(new { error = "Credenciales inválidas." });
+
+        var expiresAt = DateTime.UtcNow.AddHours(_jwtSettings.ExpirationHours);
+        var token = GenerateJwtToken(user.Id, user.Username, user.Email, expiresAt);
+
+        return Ok(new LoginResponseDto
+        {
+            Token = token,
+            ExpiresAt = expiresAt
+        });
+    }
+
+    private string GenerateJwtToken(int userId, string username, string email, DateTime expiresAt)
+    {
+        var key = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(_jwtSettings.Secret));
+        var credentials = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);
+
+        var claims = new[]
+        {
+            new Claim(JwtRegisteredClaimNames.Sub, userId.ToString()),
+            new Claim(JwtRegisteredClaimNames.UniqueName, username),
+            new Claim(JwtRegisteredClaimNames.Email, email),
+            new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString())
+        };
+
+        var token = new JwtSecurityToken(
+            issuer: "PruebaGentle",
+            audience: "PruebaGentle",
+            claims: claims,
+            expires: expiresAt,
+            signingCredentials: credentials);
+
+        return new JwtSecurityTokenHandler().WriteToken(token);
+    }
+}