feat: Add Register DTOs and SP for public registration (Refs #2)

Backend/Sql/sp_User_Register.sql

@@ -0,0 +1,26 @@
+CREATE OR ALTER PROCEDURE sp_User_Register
+    @Username NVARCHAR(50),
+    @PasswordHash NVARCHAR(255),
+    @Email NVARCHAR(100),
+    @NombreCompleto NVARCHAR(100)
+AS
+BEGIN
+    SET NOCOUNT ON;
+
+    -- Check if username already exists
+    IF EXISTS (SELECT 1 FROM Users WHERE Username = @Username)
+    BEGIN
+        THROW 50001, 'El nombre de usuario ya existe.', 1;
+    END
+
+    -- Check if email already exists
+    IF EXISTS (SELECT 1 FROM Users WHERE Email = @Email)
+    BEGIN
+        THROW 50002, 'El email ya existe.', 1;
+    END
+
+    -- Insert new user
+    INSERT INTO Users (Username, PasswordHash, Email, NombreCompleto)
+    OUTPUT INSERTED.Id, INSERTED.Username, INSERTED.Email, INSERTED.NombreCompleto, INSERTED.FechaCreacion
+    VALUES (@Username, @PasswordHash, @Email, @NombreCompleto);
+END