feat: remaining services and controllers

- ContratoService + ContratoController (CRUD, PDF data, empresa, firma)
- CotizacionService + CotizacionController (CRUD, detalle, pago, empresa)
- AlumnoService + AlumnoController (CRUD, apoderado, consultas, colegios)
- InformeService + InformeController (reportes ventas, leads, documentos)
- All services registered in DI (Program.cs)

backend/src/Ventas.API/Program.cs

@@ -26,6 +26,14 @@ builder.Services.AddScoped<LeadService>(sp =>
     new LeadService(connectionString));
 builder.Services.AddScoped<UsuarioService>(sp =>
     new UsuarioService(connectionString));
+builder.Services.AddScoped<ContratoService>(sp =>
+    new ContratoService(connectionString));
+builder.Services.AddScoped<CotizacionService>(sp =>
+    new CotizacionService(connectionString));
+builder.Services.AddScoped<AlumnoService>(sp =>
+    new AlumnoService(connectionString));
+builder.Services.AddScoped<InformeService>(sp =>
+    new InformeService(connectionString));
 
 var jwtSecret = builder.Configuration["Jwt:Secret"] ?? "default-dev-secret-change-in-production";
 var jwtExpiration = int.Parse(builder.Configuration["Jwt:ExpirationMinutes"] ?? "30");