fix: complete repository layer and refactor services

- Added ContratoRepository, CotizacionRepository, AlumnoRepository, UsuarioRepository (Infrastructure)
- Added IInformeRepository + InformeRepository for report SPs
- Refactored ContratoService, CotizacionService, AlumnoService, UsuarioService to use repos via DI
- Refactored InformeService, EmpresaReportService to use IInformeRepository
- Updated Program.cs DI to wire repos correctly with connection string factory
- Build: 0 errors

backend/src/Ventas.Infrastructure/Repositories/CotizacionRepository.cs

@@ -0,0 +1,44 @@
+using Dapper;
+using Npgsql;
+using Ventas.Core.Interfaces;
+
+namespace Ventas.Infrastructure.Repositories;
+
+public class CotizacionRepository : ICotizacionRepository
+{
+    private readonly string _connectionString;
+
+    public CotizacionRepository(string connectionString)
+    {
+        _connectionString = connectionString;
+    }
+
+    public async Task<string> PersonaIngresarAsync(string apoderado, string vendedor, int solicitudDescuento, int descuento, int tipoDescuento, string fecha, int alumnos, int curso, int monto, string validez, int leadId)
+    {
+        using var connection = new NpgsqlConnection(_connectionString);
+        return await connection.QuerySingleOrDefaultAsync<string>(
+            "sige_sam_v3.IngresarCotizacionLead",
+            new { apoderado, vendedor, solicituddescuento = solicitudDescuento, desctoid = descuento, tipodesctoid = tipoDescuento, fecha, alumno = alumnos, cantidad = curso, monto, validez, leadnum = leadId },
+            commandType: System.Data.CommandType.StoredProcedure) ?? "ok";
+    }
+
+    public async Task<string> PersonaPagarAsync(int cotizacionId)
+    {
+        using var connection = new NpgsqlConnection(_connectionString);
+        await connection.ExecuteAsync(
+            "sige_sam_v3.PagarCotizacion",
+            new { cotizacionid = cotizacionId },
+            commandType: System.Data.CommandType.StoredProcedure);
+        return "ok";
+    }
+
+    public async Task<string> DesactivarAsync(int cotizacionId)
+    {
+        using var connection = new NpgsqlConnection(_connectionString);
+        await connection.ExecuteAsync(
+            "sige_sam_v3.DesactivarCotizacion",
+            new { cotizacionid = cotizacionId },
+            commandType: System.Data.CommandType.StoredProcedure);
+        return "ok";
+    }
+}