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.Infrastructure/Repositories/UserRepository.cs

@@ -0,0 +1,89 @@
+using System.Data;
+using Dapper;
+using Microsoft.Data.SqlClient;
+using Microsoft.Extensions.Configuration;
+using PruebaGentle.Core.Entities;
+using PruebaGentle.Core.Interfaces;
+
+namespace PruebaGentle.Infrastructure.Repositories;
+
+public class UserRepository : IUserRepository
+{
+    private readonly string _connectionString;
+
+    public UserRepository(IConfiguration configuration)
+    {
+        _connectionString = configuration.GetConnectionString("DefaultConnection")
+            ?? throw new InvalidOperationException("Connection string 'DefaultConnection' not found.");
+    }
+
+    private IDbConnection CreateConnection() => new SqlConnection(_connectionString);
+
+    public async Task<User> CreateAsync(User user)
+    {
+        using var connection = CreateConnection();
+        var result = await connection.QuerySingleAsync<User>(
+            "sp_User_Create",
+            new
+            {
+                user.Username,
+                user.PasswordHash,
+                user.Email,
+                user.NombreCompleto
+            },
+            commandType: CommandType.StoredProcedure);
+
+        return result;
+    }
+
+    public async Task<User?> GetByIdAsync(int id)
+    {
+        using var connection = CreateConnection();
+        return await connection.QuerySingleOrDefaultAsync<User>(
+            "sp_User_GetById",
+            new { Id = id },
+            commandType: CommandType.StoredProcedure);
+    }
+
+    public async Task<IEnumerable<User>> GetAllAsync()
+    {
+        using var connection = CreateConnection();
+        return await connection.QueryAsync<User>(
+            "sp_User_GetAll",
+            commandType: CommandType.StoredProcedure);
+    }
+
+    public async Task<User?> UpdateAsync(User user)
+    {
+        using var connection = CreateConnection();
+        return await connection.QuerySingleOrDefaultAsync<User>(
+            "sp_User_Update",
+            new
+            {
+                user.Id,
+                user.Username,
+                user.Email,
+                user.NombreCompleto
+            },
+            commandType: CommandType.StoredProcedure);
+    }
+
+    public async Task<bool> DeleteAsync(int id)
+    {
+        using var connection = CreateConnection();
+        var affected = await connection.ExecuteAsync(
+            "sp_User_Delete",
+            new { Id = id },
+            commandType: CommandType.StoredProcedure);
+        return affected > 0;
+    }
+
+    public async Task<User?> GetByUsernameAsync(string username)
+    {
+        using var connection = CreateConnection();
+        return await connection.QuerySingleOrDefaultAsync<User>(
+            "sp_User_GetByUsername",
+            new { Username = username },
+            commandType: CommandType.StoredProcedure);
+    }
+}